The Motor Shield 2A plugs in on top of the Arduino Uno, and the LCD Keypad plugs in on top of the motor shield.
There is better information about the 4 Channel Line Tracking Sensor at uctronics.com where you can see that its main board (with the LM339 chip) connects to the Arduino via six lines: Vcc, Gnd, Out1, Out2, Out3, Out4. That main sensor board does not mount on the stack, but instead can be located whereever is convenient and leaves its four comparator-level pots accessible for adjustment. Mount the four small boards for the tracking module as convenient and necessary for line following.
For connecting the six lines from the sensor main board to the Uno, you probably will need to solder two headers (rows of pins) to a board in the stack. Using right-angle headers (see below) will allow the pins to come out of the side of the stack without interfering with other layers of the stack. One of the headers should let you connect sensor Out1...Out4 digital outputs to four otherwise unused analog or digital pins, and the other header should let you connect Vcc and Gnd.
