Simple, examine all your choices,
conduction: process by which heat is transmitted through collisions between neighboring molecules. Ex: Frying Pan on Stove.
convection: heated molecules move from one place to another taking the heat with them. Ex:Sun heating Earth, air cools, etc.
radiation:electromagnetic waves that cause ionization.
Out of all these, the only logical answer is A. Conduction.
The heater is heating the water in the fish tank, unmoving, it goes no where.