A' Stallion' is a male-horse while a 'More ' is a female horse
Therefore, the relationship is male : Female. In options (a), (c) and (d) this relationship is inverted. In option (b) 'Ram', a male sheep, proceeds 'ewe' a female sheep.

'Mycology' is the study of 'Fungi', in the same way 'Pedology' is the study of 'Soil'.

'Alleviate' means 'to Reduce ' and 'Aggravate' means 'to intensify'. So, it is a clear opposite analogy and it is similar to 'Elastic' and 'Rigid' which are mutually opposite.

Second is the young - one of first.

Muddy , Benevolent and kind are mutually synonyms, so muddy is the best option which is similar to unclear in meaning.

As, (1/9)² = 1/81
Similarly, (1/13)² = 1/169

As, 25 : 625 ? 25 is common in both and 6 is added before it.
Similarly, 35 : 635 ? 35 is common in both and 6 is added before it.

As, 1234 : 10
1 + 2 + 3 + 4 = 10
Similarly, 6789 : 30
6 + 7 + 8 + 9 = 30

As, 66 - 2 = 64 ? ? 64 = 8
Similarly, 38 - 2 = 36 ? ? 36 = 6

Taking option (c),
As sum of digits of 583 = 5 + 8 + 3 + = 16
and sum of digits of 488 = 4 + 8 + 8 = 20
Similarly, sum of digits of 293 + 2 + 9 + 3 = 14
and Sum of digits of 378 = 3 + 7 + 8 = 18
Here, difference of both are (20 - 16) = (18 - 14) = 4
? ? = 378