How to print the data in an array in C?

The conversion specification %.00f requests 0 digits after the decimal point, so that's what you get. If you want 2 digits after the decimal point, then use %.2f (or maybe %6.2f).

To fix the point that Matteo Pacini made in his comment, and deal with miscellaneous other issues too, you could use a variant of your code similar to this:

#include <stdio.h>

int main(void)
{
    int grades[20];
    char names[20][20];
    int a, b, f, high, g, loc;
    float avg, tot = 0;
    for (a = 0; a < 20; a++)
    {
        printf("Input Name %d: ", a+1);
        if (scanf("%19s", names[a]) != 1)
            return 1;
    }
    for (b = 0; b < 20; b++)
    {
        int e;
        printf("Input Marks %d: ", b+1);
        if (scanf("%d", &e) != 1)
            return 1;

        if (e >= 0 && e <= 100)
            grades[b] = e;
        else
        {
            printf("OUT OF RANGE.PLEASE INPUT A VALID NUMBER.\n");
            b--;
        }
    }
    putchar('\n');

    for (a = 0; a < 20; a++)
        printf("%-20s %3d\n", names[a], grades[a]);

    for (f = 0; f < 20; f++)
        tot = tot + grades[f];

    printf("Total is %6.2f\n", tot);
    avg = tot / 20.00;
    printf("Average is %6.2f\n", avg);

    high = grades[0];
    loc = 0;
    for (g = 0; g < 20; g++)
    {
        if (high < grades[g])
        {
            high = grades[g];
            loc = g;
        }
    }

    printf("The average is %6.2f\n", avg);
    printf("The highest grade is %d\n", high);
    printf("The name of the person who has the highest grade is: %s\n", names[loc]);

    return 0;
}

Given sample input data like this (file ga.marks), where the data was generated by two programs:

Student-01
Student-02
Student-03
Student-04
Student-05
Student-06
Student-07
Student-08
Student-09
Student-10
Student-11
Student-12
Student-13
Student-14
Student-15
Student-16
Student-17
Student-18
Student-19
Student-20
49
27
47
46
33
84
63
51
61
91
82
60
39
57
65
60
19
88
47
61

This is the output from running the program (named ga) as: ga < ga.marks (because I am not going to sit around typing 20 names and 20 numbers as input to a program):

Input Name 1: Input Name 2: Input Name 3: Input Name 4: Input Name 5: Input Name 6: Input Name 7: Input Name 8: Input Name 9: Input Name 10: Input Name 11: Input Name 12: Input Name 13: Input Name 14: Input Name 15: Input Name 16: Input Name 17: Input Name 18: Input Name 19: Input Name 20: Input Marks 1: Input Marks 2: Input Marks 3: Input Marks 4: Input Marks 5: Input Marks 6: Input Marks 7: Input Marks 8: Input Marks 9: Input Marks 10: Input Marks 11: Input Marks 12: Input Marks 13: Input Marks 14: Input Marks 15: Input Marks 16: Input Marks 17: Input Marks 18: Input Marks 19: Input Marks 20: 
Student-01            49
Student-02            27
Student-03            47
Student-04            46
Student-05            33
Student-06            84
Student-07            63
Student-08            51
Student-09            61
Student-10            91
Student-11            82
Student-12            60
Student-13            39
Student-14            57
Student-15            65
Student-16            60
Student-17            19
Student-18            88
Student-19            47
Student-20            61
Total is 1130.00
Average is  56.50
The average is  56.50
The highest grade is 91
The name of the person who has the highest grade is: Student-10

Note that the prompts are counter-productive for input from file. The code echoes its input at the end of the input. It does the calculations etc. I didn't bother to tune the data to produce a mean score of 48.95.

Use printf("%.2f",avg) for 2 digits after the decimal point. You are using printf("%.00f",avg) which means 0 digits after the decimal point.

Your calculations are correct. Just the printing format is messed up.

Use the code below rather than %.00f:

 %.2f

To fix your name problem you have many ways. One possibility is as below:

char names[20][50];

This means you have 20 names with each length 50 max.

scanf("%s", names[a]);

Also you can change your scanf as above. Remove &.