Question

states that all the continents were once a continent and have since drifted apart.
select
select
wegener theory
plate tectonic theory
continental drift theory
pangaea theory

Explanation:

The described concept, that all continents were once a single landmass and drifted apart, is the core of the Continental drift theory. Wegener is the scientist who proposed this theory, plate tectonic theory expanded on it with more details about crustal plates, and "Pangea theory" is not a standard scientific term (Pangea is the name of the supercontinent).

Answer:

Continental drift theory