The sign that is not specifically mentioned in the Bible as a convincing sign to the Israelites that Moses was sent by God is:

D. Flowers growing from Moses' rod.

The other signs (A. Moses' rod becoming a serpent, B. Moses' hand becoming leprous, and C. River Nile becoming a pool of blood) are documented in the Book of Exodus as miraculous events that occurred to validate Moses' divine mission to lead the Israelites out of Egypt.