Why did becquerel expose uranium crystals to sunlight in his initial experiment?

The correct answer is that he wanted to show that the crystals emit X-rays.

Explanation:

At the time of the investigation of the newly discovered X-rays, Henri Becquerel studied salts of uranium that got influenced by light. He found that the salt of uranium continuously discharged penetrating radiation that is shown on a photographic plate. Afterward, Becquerel showed that the radiation discharged by the uranium salts exhibited some features of X-rays.  

Thus, in his initial experiment, Becquerel exposed uranium crystals to sunlight in order to demonstrate that the crystals discharged X-rays that would illustrate the patterns witnessed on the photographic plates encountered with uranium salts.