What's the word?

Katy's brother isn't short.​ ​ (a)  

• His brother is tall
• Her brother is tall

Mr. and Mrs. Miller's house isn't cheap. ​ (a)  

• Their house is expensive.
• His house is expensive

Peter's sister isn't single. ​ (a)  

• His sister is married.
• Her sister is married

Ms. Allen's neighbors aren't quiet. ​ (a)  

• Her neighbors are noisy
• Their neighbors are noisy

Their dog's name isn't Spot. ​ (a)  

• Its name is Apollo.
• Their name is Apollo.

Mrs. Brown's car isn't large. ​ (a)  

• Her car is small.
• His car is small.

Paul's bicycle isn't new. ​ (a)  

• His bicycle is old.
• Its bicycle is old.

Mr. and Mrs. Carter's son isn't single. ​ (a)  

• Their son is married.
• Her son is married.