Penguins seem comfortable in the coolest habitats which identifies the predicate adjective

Comfortable

Explanation:

One type of the adjectives is predicate adjective. A predicate adjective is an adjective which follows a linking verb and modifies or refers back to the main subject of that linking verb.

It usually is used just before the noun it modies.

Here in this example:

'Penguins seem comfortable in the coolest habitats', comfortable is the predicate adjective which follows the linking verb 'seem' and modifies the noun.