When drive-ins were at the height of their popularity in the

[RonPurewal]

‘some 4,000 existed in the United States’ is a non essential modifier. Hence if it is omitted

very much incorrect. that's a complete sentence, so, by definition it can't be a modifier. (overall, the sentence has the construction [complete sentence] + "but" + [other complete sentence].)

[AnirbanM72]

Hi Experts,
I can eliminate options C,d And E as amount refers to uncountable quantities. I can discard option a as less refers to uncountable quantities and so I can rightly pick B as an answer. But I have a query. According to fanboys rule, comma + but should produce an independent clause. Option B states, but today that there are fewer than one-quarter as many. So, is it violating fanboy rule?

[Sage Pearce-Higgins]

Where did you find this "fanboy rule" to which you refer? We often use conjunctions to link independent clauses, but not always. This is especially the case with ellipsis, i.e. part of the sentence is left out as it would just be obvious repetition. See Ron's post above about this.

[TheGreatTony]

THANKS so much for eliminating my confusion!

Ron, you are the life-saver！

[Sage Pearce-Higgins]