Answer:
intransitive
Step-by-step explanation:
a transitive verb only makes sense if it is directly followed by an object in the sentence (e.g. his sister sent the postcard, where "sent" is the verb and "the postcard" is the object the action is exercised upon. by definition, "transitive" means "passing over to or affecting something else," (Dictionary.com), meaning that the action is happening to an object.)
an intransitive verb is not followed by an object in the sentence (e.g. "under" is not an object, it's a preposition. the definition of "intransitive" is the precise opposite of that of "transitive," as you may have guessed. to clarify, the action just happens, but not to something else. another example: the cat purred while being pet, where "purred" is the verb, but the action was not exercised upon something else - the cat just purred. just like with the creaking barn timbers here.)
a linking verb simply connects the subject of the sentence to more information about the subject. they attach to the rest of the sentence and don't show action, as you would expect verbs to do.