Question

when molecules of glucose and fructose combine to form sucrose, they do so by...

Answer 1

In a dehydration reaction. An oxygen branching off one of the sugars, probably glucose (really doesn't matter which order), will form water with a hydroxyl (-OH) group from the other sugar, which would be fructose in this case. The water then falls out and the two sugars are linked.

Answer 2

^that's correct. although it matters which oxygen is being used to form the bond.