The two oil companies agreed to merge their refining and marketing ope

In this case, you must understand that C, D, and E cannot be the answers as the word 'which' cannot refer to a specific word as given in the sentence. The word "Which" intends to refer to the entire clause or to the merger (noun) ... but there is no noun to which the word 'which' can directly refer as 'merge' is a verb.

The second clause is a result of the first... in all cause-effect relationships, the GMAT prefers the -ING form of the verb after the comma for the effect part.

Also, this ING form is applicable when one of the following is correct:

The second action is a result / effect / offshoot of the first; … Cause and effect.
The second action is a part of the first and not a separate action;
The second action is dependent on the first;
The second action is sub-ordinate to the first;
The second action is embedded into the first action.
The second action can’t happen without the first action independently
The second action explanation (how / why the first action happens) of the first action … etc.

WE USE the second verb in the -ING form…

Also, A is wrong for the wrong idiom. Whenever we intend to denote the purpose, intention, desire etc., "to + verb" is considered idiomatically correct. So, A can be eliminated because it uses the wrong idiom 'for controlling'.

B is the best choice. In B, the use of the word 'would' is completely justified as it is used for a future event when viewed with respect to the past.

B: Correct