つくえの ＿＿＿ ＿＿＿ ＿★＿ ＿＿＿ あります。

Translation: There are 5 books on the desk. - Sentence assembly: The correct order is 'つくえの [① 上に] [② 本] [★ ③ が] [④ ５冊] あります。' - Grammatical rule: The existence pattern is [Location] + に + [Subject] + が + [Quantity] + あります. Therefore, the sequence is: 'つくえの 上に 本 が ５冊 あります'. - The third slot (★) is filled by 'が' (Choice 3). - Therefore, the answer is 'が'.