word order of "become," "sufficiently", "advanced" in this sentence

Question

So this is still a mystery to me. Can someone help me please? This is the question:

It was only when electric motors ______________ to become portable that vacuum cleaners became common household items.

1) became advanced sufficiently

2) sufficiently became advanced

3) had sufficiently become advanced

4) had become sufficiently advanced

And please tell me why the 3 wrong answers are wrong. Also, can we put an adverb like "sufficiently" between "had" and "become"? Does the answer to the last question only apply to this sentence or is it always like that? And one last thing, please choose one of the answers given. This was a test, so no other answers were available.

Answer 1

The fourth one is the correct one. Reasoning: First of all, let us focus on why the fourth alternative is the correct choice. In the sentence, we are talking about the order of two events in which the second occurred after the first one had already happened because the second event was conducive only due to the happening of the first one. Hence, we need to use past perfect tense here leading to the cancellation of the first two options.

Now the question is about "advanced sufficiently" or "sufficiently advanced". Here we are trying to focus on the word "advanced" since if the electric motors hadn't become advanced, we wouldn't have seen portable vacuum cleaners. In order to focus the word "advanced", we need to put the adverb "sufficiently" just before it (since we are interested in the advancement of the technology).

P.S.: The third law of British science fiction writer Arthur C. Clarke states: "Any sufficiently advanced technology is indistinguishable from magic".(https://en.wikipedia.org/wiki/Clarke%27s_three_laws)