Answer:
The answer is correct.
La cena estaba preparada. The past participle used as an adjective is preparada.
Step-by-step explanation:
Verbs in past participle working as adjectives end with ado or ido, like preparada, and they usually have before them the verb to be (estar).
The past participle used as an adjective modifies the noun dinner (cena). Notice that we can replace this adjective with another to alter the state of dinner. For example, we can say, la cena estaba congelada (dinner was frozen).
Also, preparada coincides in number and person with the noun that it is modifying. La cena is a singular feminine noun, and the correct adjective form for it is the one ending in ada, which in this case is preparada.