Which shows a rational number plotted correctly on a number line?

The second illustration is correct. Note that the given number, -21/2, is between -24/2 and -20/2. Identifying and using the LCD (here, 2) makes the comparison and placement much easier than they would be otherwise.

We know that any negative number is always represented to the left of zero on the  number line.

In first graph:

The number -3.5 is represented to the right of zero.

It is represented in between 2 and 4.

But we know that -3.5 lie between -4 and -2.

Hence, option: 1 is incorrect.

In second graph:

 The number -21/2 is  a negative number and lie between -11 and -10

              Hence, option: 2 is correct.

In third graph:

The number -7/2 lie between -4 and -3.

But in the graph it is plotted to the left of -4 whereas it should be to the right of -4.

Hence, option: 3 is incorrect.

In fourth graph:

The number 9/2 is equal to 4.5

and it must lie between 4 and 5.

i.e. it should be plotted to the right of 4.

Hence, option: 4 is incorrect.