Answer: D. Cockatiels enjoy summer sunshine.
Step-by-step explanation: an adjective is a word or a group of words that modifies, describes or gives attributes to a noun (or a noun phrase). From the given options, the one that contains an adjective is the sentence corresponding to option D: Cockatiels enjoy summer sunshine, the adjective is "summer" and it modifies (describes) the noun "sunshine," in the other sentences, the word "summer" acts like the object of a preposition.