Why does rama give sugriva a necklace made of flowers?

To tell him apart from Baali.

Explanation:

Sugriva (or Sugreeva) and Bali are two brothers that appear in the Ramayana. Baali was the older brother, while Sugriva was younger. They are both sons of Riksh Raaj, who was turned into a woman. He became a beautiful woman, and both Soorya and Indra wanted to have her. As they did, Baali was fathered by Indra, while Sugriva was fathered by Soorya. Rama gives Sugriva a necklace made of flowers in order to tell him apart from Baali.

because it is questonabile