The reason is ---
(a) (108 ÷ 3 = 36) (b) (69 ÷ 3 = 23)
(c) (42 ÷ 3 = 14) (d) Can not follow

Except option (a), all are means to buy only.

aacdff

Fathom its one of the unit of lenght and rest are adjective defining states or shape of any substance.

Except (c) all are written from right to left.

MONR

Adding all the numbers of the different options gives a value which is divisible by 3. Except option (d)
(a) 6+7+6+2+6 = 27 divisible by 3
(b) 8+4+1+2+9 = 24 divisible by 3
(c) 3+2+4+1+8 = 18 divisible by 3
(d) 4+7+6+3+2 = 22 not divisible by 3

Except Eye, rest all are abdominal organ.

QNKH

Except (c) all are perfect square. i. e. 11² = 121,
12² = 144, 13² = 169.