Answer:

a.

Explanation:

We know that the larger the electronegativity difference of bonded atoms, the larger will be the dipole moment.  

Element | Electronegativity

C         |2.55

O         |3.44

H         |2.2

N         |3.04

S         |2.58

For a)  O-H

Electronegativity difference=

For b)  S-H

Electronegativity difference=

For c)  N-H

Electronegativity difference=

For d)  C-H

Electronegativity difference=

Since has the largest electronegativity difference this implies that has the largest dipole moment.

Hence, the answer is a.