What is the relation between Glucose and Mannose?

Epimers

The correct answer is option 3. Epimers.

Epimers are a specific type of stereoisomer that have multiple stereocenters but only differ from one another by the configuration at one of the stereogenic centers (Chiral carbon centers). Glucose and mannose differ from each other by the configuration at C-2 (the second carbon atom). So, they are epimers of each other.