An array of tax incentives has led to a boom in the construction of ne

Dear anurag16,

I'm happy to respond.

The short answer to your question is that the word "so" has more than one use.

One use of "so" is as a subordinate conjunction. For example:
The student walked in late, so the teacher yelled at him.
Notice that this sentence has only one independent clause.
independent clause = the student walked in late
subordinate clause = so the teacher yelled at him
Notice that in any subordinate clause that begins with a subordinate conjunction, it is true 100% of the time that if we were to remove the subordinate conjunction, what follows could function as an independent clause. Nevertheless, as long as that subordinate conjunction is there, we are not allowed to count it as an independent clause. See:
GMAT Grammar: “On a White Bus” with Subordinate Conjunctions

In this SC problem, we are NOT using "so" as a subordinate conjunction.

A completely different use of "so" is as an intensifier for an adjective, in the structure:
so [adjective] that . . .
The Antarctic winter is so cold that frostbite occurs almost instantaneously.
Notice, it would be 100% correct and even quite sophisticated to change the word order:
So cold is the Antarctic winter that frostbite occurs almost instantaneously.
The word "so" has to come immediately before the adjective it intensifies, but there's no rule that the word "that" has to come right after. In fact, this latter wording is typical of very high quality writing because rhetorically it emphasizes both the adjective and the logical tension in the sentence. We hear "So cold is the . . . " and it immediately creates the question in the reader "how cold is it?" The sentence creates that tension and satisfies it only in the second half: this is a brilliant rhetorical device that elicits the reader's curiosity and draws the reader to read the entire sentence with that much more interest and attention. It is a particular example of the way in which a skilled writer might hook a reader's attention and drive that attention through the length of the piece.

This is precisely the rhetorical device that appears in (A), the OA.
. . . so abundant has capital been for commercial real estate that investors regularly scour the country for areas in which to build.
This is an independent clause. Perhaps this would be more clear if we were to change the word order to a more prosaic, pedestrian, and un-dramatic version, the "white bread only" phrasing of the information:
. . . capital for commercial real estate has been so abundant that investors regularly scour the country for areas in which to build.
This latter version makes the grammatical structure clearer to folks struggling to see this structure, but ultimately, that is not how language is used in the real world. In the real world, the artful use of language involves using all the elements of language together to communicate one's meaning most powerfully, and this is what version (A) does well.

Of course, what is so brilliant about using this sophisticated rhetorical construction after a semicolon is that all the students who mistake this for the subordinate conjunction use of "so" will think it's wrong! Among other things, it's a brilliant test about the difference between two very different uses of "so"! Official questions are always so brilliantly designed! As someone who writes questions professionally, I am continually in awe of the quality of the official questions!

Does all this make sense?
Mike