昨日、公園で猫___見ました。
Q. Which particle correctly marks 猫 as the object of 見ました?
Translation
Yesterday I saw a cat in the park.
Explanation
見ました (saw) is a transitive verb, and 猫 is the thing being seen — the direct object. Direct objects take を. は would make 猫 a topic ('as for the cat...'), and が marks subjects, not objects.