Heat transfer by conduction occurs when
molecules
(1) flow as currents through liquids
(2) form waves that travel through space
(3) become less dense and rise

It is none of the three. Conduction occurs when a heat source directly is touching an object. 

Explanation: Number 1 is convection. Key term: convection currents. Number 2 is radiation, as radiation can pass through space. Number 3 is just explaining a part of the convection current.

The answer is b because a,c,and d are opinons