Final answer:
The limitation of the interference theory by McGeoch and McDonald is that the strength of interference depends on the similarity between the two sets of materials.
Step-by-step explanation:
The limitation of the interference theory by McGeoch and McDonald is that the strength of interference depends on the similarity between the two sets of materials (Option C). It is incorrect to say that proactive interference is always stronger than retroactive interference (Option A) or vice versa (Option B). Additionally, it is incorrect to say that interference theory has no limitations (Option D).