Couple of things
a) The sentence is attempting to join two clauses - ( this can be either done by using something such as which, that or by using a conjunction such as and, but)
The problem with the sentence is that it is trying to join the two clauses w/o using a conjunction - something called a comma splice. The only allowed conjunctions to join two sentences using a comma are For, And, Nor, But, Or, Yet, So ( can be memorized easily using the anagram FANBOYS)
The answer choices are attempting to join the sentence using a which - while that is correct, however, which ALWAYS refers to a noun ( as opposed to the act of buying / investing that we have here). In the sentence, which is referring to the million dollars. It is not the million dollars, but the purchase that shall make the company the largest manufacturer. So the options with a ,which are also crossed out. Leaving E.
To grasp this concept, I recommend Googling Comma Splice and understanding the basics from there.