In “the seafarer” the phrase “summer’s sentinel,” meaning a cuckoo, is an example of a. a kenning. c. a scop. b. a predicate. d. an exile.

The correct answer is a. a kenning.

A kenning is a stylistic element in which compounds with figurative meaning are used instead of simple and single words, like you have there. This was very common in 10th, 11th, century poetry. For example, traditionally warriors were sometimes mentioned as "feeders of eagles" because it sounds better to the poet.