1. INK

SINK

-> extra letter: S

2. EAR

EARN

-> extra letter: N

3. ANT

AUNT

-> extra letter: U

4. BAT

BOAT

-> extra letter: O

5. HER

HEAR

-> extra letter: A

6. RAIN

TRAIN

-> extra letter: T

7. HAD

HARD

-> extra letter: R

8. ALL

TALL

-> extra letter: T

9. MEN

MEAN

-> extra letter: A

=> ASTRONAUT