More and more couples wait before trying to start a family, the

Clear B!

Got there by POE.
trying and starting in Option C, does not sound right.
Although, when added to the sentence in Option D creates
an unnecessary contrast and changes the original meaning of the sentence.
In Option E, being is not sure properly and the sentence is just awkward.
Option A is not clear because "As X, Y" clearly communicates the meaning of the sentence - X leads to Y

KAPLAN OFFICIAL EXPLANATION

STEP 1: READ THE ORIGINAL SENTENCE CAREFULLY, LOOKING FOR ERRORS
This sentence contains two complete clauses, each with its own subject and verb. The subject of “wait” is “couples,” and the subject of “is increasing” is “age.” Two complete clauses should be joined with a conjunction, but this sentence has no conjunction and is thus a run-on. The correct answer will provide the sentence with a conjunction. Moreover, the logical relationship between clauses is one of cause and effect. The delay in starting a family in the first clause logically leads to an increase in the average age. So look for a choice that establishes that relationship. Finally, note that the junction between the clauses is not underlined, so the connector word cannot go there; it will have to place the first clause into the correct relationship with the second.

STEP 2: SCAN AND GROUP THE ANSWER CHOICES
Choices (B) and (C) introduce “[a]s” at the beginning of the sentence. Choice (D) substitutes “[a]lthough,” and (E) uses “being that.”

STEP 3: ELIMINATE CHOICES UNTIL ONLY ONE REMAINS
Eliminate (A) for omitting a connecting word. “Being that” is a weak, awkward construction, and on the GMAT an answer choice containing it will be wrong; eliminate (E). Choice (D) adds the conjunction “[a]lthough,” which indicates a contrast and therefore doesn’t make logical sense. (B) and (C) both start with “[a]s,” meaning “because,” which does make logical sense as a conjunction for this sentence. (C) changes “trying to start” to “trying and starting,” which is idiomatically incorrect. The correct answer is (B), which adds an appropriate conjunction and does not create any new errors. Read (B) back into the sentence to confirm:

As more and more couples wait before trying to start a family, the average age of first-time parents is increasing.

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