The difference of two like fractions can be obtained following these steps:
Step 1: Find the difference of numerators.
Step 2: Retain the (common) denominator.
Step 3: Write the fraction as:
So,
Result of step 1
/
Result of step 2
So,
13
/
21

5
/
21
=
13  5
/
21
Therefore, option (d) is correct.