Final answer:
The best way to combine sentences 2 and 3 is Option D, which uses a gerund phrase and appropriate punctuation for clarity and conciseness.
Step-by-step explanation:
The most effective way to combine sentences 2 and 3 is by choosing an option that maintains proper sentence structure and logical flow. Option D, 'By his early twenties, Hitchcock was already working in the studio business, going from simple draftsman to assistant director in less than three years.' is the correct choice because it effectively combines the two sentences without creating a run-on sentence, fragment, or comma splice, and it uses a gerund phrase to indicate the progression of Hitchcock's career succinctly.