so...that idiom is needed - so this eliminates B & E. In C we dont have what is researched and without that there will be an ambiguity in ITS in non-underlined part..In D the subject is the research and testing and this does not agree with ITS in the non-underlined part.....Ans is A.

B. Ignore it. SO....THAT is correct idiom.
C. Ignore it. "What was tested and researched so secretly?" is never answered.

D. Ignore it. Research and Testing should be in parallel.

E. Ignore it. See the last underlined part...."as to make it consumers were unaware." doesn make any sense.

Hence, A is correct...Researched and tested is correctly used in parallel.
So...that is used in cause-effect relation.

Option A is the correct answer because it follows proper parallelism.' Research and tested' the two follow each other and were used correctly in a perfect order.

Option B.This option is incorrect, it is very awkward to start a sentence with 'So secretly' there are better ways to start a sentence.Thus option B has an awkward construction.

Option C. Sound so ridiculous because it does not even make reference to the subject which is 'the product'.The product is supposed to be the centre of attraction.

Option D. 'been so secret' but it has reached the target market which is 'the stores' therefore, it has stopped being secret.Thus, it does not have to be seen as being secret anymore.

Option E. 'A product'? This is improper to define a particular product. A 'the' would have been better because 'the' is a definite article to describe something unique

Hello!

To answer your question about B:

It sound awkward to readers to start a sentence with a coordinating conjunction (for, and, nor, but, or, yet, so). Sometimes, you can get away with starting a sentence with the word "so," but only if it's being used casually as an interjection or for emphasis. Here's an example of how you can use it correctly:

So, what time are we leaving for the movie?

So, it's fine to use them for emphasis (usually in an informal or casual tone). Yet, you should avoid starting sentences with them in any other situation.

(See what I did there?)

I hope this helps! I'm available if you'd like any follow up.