Which letter is silent?

Which letter is silent?

Select the silent letter

Knife
1. k
2. f
Through
1. g
2. g+h
3. h
Plumber
1. m
2. b
Friend
1. d
2. i
Night
1. g
2. h
3. g+h
Doubt
1. b
2. t
3. b+t
Chemistry
1. c
2. h
Half
1. l
2. h
Know
1. w
2. k
Psychology
1. s
2. p
Marriage
1. i
2. a
Thumb
1. h
2. m
3. b
Knee
1. k
2. n
Answer
1. w
2. n
Building
1. g
2. u
Guest
1. u
2. t
Could
1. l
2. d
Write
1. i
2. w
Listen
1. n
2. t
Ghost
1. s
2. g