Answer: D (3 or 4). Beryllium hydroxide is amphoteric, so it reacts with both acids and alkalis. Calcium hydroxide is basic, so it reacts with acids only. To distinguish them you must use an alkali.
With aqueous KOH (reagent 3) or aqueous NaOH (reagent 4), the amphoteric beryllium hydroxide dissolves to give a clear solution, while calcium hydroxide does not react — a clear difference.
The two acids, HCl (1) and HNO₃ (2), both dissolve both hydroxides, so neither acid distinguishes them. Only an alkali works, so the answer is 3 or 4.
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