Which of the following is an organic molecule
a) oxygen gas
b) sucrose
c) iron oxide
d) water

Option B - Sucrose

Explanation: Any compound containing carbon atoms covalently bound to other atoms is referred to as an ORGANIC COMPOUND or MOLECULE.

From the options provided, Oxygen gas (O2) does not have any carbon atom, so it is not an organic molecule; so also iron oxide (Fe2O3) and water (H2O).

Sucrose, however, with formula C12H22O11, consists of 12 carbon atoms bound to 22 hydrogen atoms, and 11 oxygen atoms.

So, Sucrose is the right answer.