In 'Completing The Analogous Pair', two words are given. These words are related to each other in some way. Another word is also given. The candidate is required to find out the relationship between the first two words and choose the word from the given alternatives, which bears the same relationship to the third word, as the first two bear.
Cattle : Herd :: Sheep : ?
Herd is a group of cattle. Similarly, flock is a collection of sheep.
So, answer is (a) Flock.