Answer:
a. will be
✓ My sister will be 21 years old next Sunday.
✗ My sister will have 21 years old next Sunday.
✗ My sister was 21 years old next Sunday.
The sentence says next Sunday, meaning in the future. 'Will be' means that she isn't 21 yet, but that she is going to be soon. The words 'will have' doesn't make sense in this sentence, and 'was' means in the past, or it already happened. However, she isn't 21 yet.
'Will be' makes the most sense.
