We need something in contrast with single-handedly, "collaborative" and "synergetic" are the right picks.

Answer B and C

Explanation

The pivot word while tells you that the later contributions were achieved in a manner opposite to “single-handedly.” You need two words that mean “cooperative.” Note that trap answers solitary and unilateral are the opposite of what you need, and collusive has the wrong spin — to collude is to “cooperate for illegal or fraudulent purposes.”

Hence options collaborative, synergetic are best fit!

One must pay heed to the word WHILE-- one must anticipate that there is going to be a shift somewhere in the sentense which indeed took place after the ,.

the clue- SINGLE handedly got reversed - and hence the asnwers B and C

First word 'While' suggests contrast in two sentences. So, the opposite of 'single handedly' is the answer. B && C

Collaborative and synergistic

kudos

kudos

My process:
solitary -- we have an "although," meaning we want something that means the opposite of "on his own." This doesn't fit the bill.
collaborative -- this works. It contradicts "on his own."
synergetic -- this works. It contradicts "on his own."
exegetic -- no clue what this meant.
unilateral -- "one-sided," which could suggest he did things on his own. No.
collusive -- no clue what this meant.

The structural clue "while" creates a contrast between the two parts of the sentence separated by a comma. And since the first part states that he devised his theory single-handedly in the early days the second part should point to a more cooperative work later in his career. Therefore the words are collaborative and synergetic.