Here, we need to pay attention to the language use in terms of phrases, clauses and/or sentences in order to pick the most suitable response.

Option A - INCORRECT.
We deal with Adjectival clause in the underlined words. When you use the words, ''portable, or having a greater chance of happening''. It would be noticed that such event that hasn't occurred cannot be used side by side with the verb ''was'' which is in the past tense. It means that the parts in A are not complementary.

Option B - CORRECT.
Note the keywords been applied here, ''more than'' which are comparative forms of adjectives used to compare between two things. Since the subject dealt with events that has happened in the past it is only appropriate to use the verb ''began'' which is a complementary past tense of ''begin'' to present our argument. This option is the mist CORRECT option.

Option C - INCORRECT.
Here, the verb part used is not in consonant with the time of event. ''Beginning'' in this context would mean a present continuous verb action which contradicts the authors viewpoint that this event happened in the EARLIEST time.

Option D - INCORRECT.
This is terribly wrong in grammatical construction. Firstly, using ''more than likely'' ought to have been accompanied with the preposition ''to'', and secondly, it must have the modal auxiliary verb ''have'' before you can use the past participle verb ''begun''. All the parts of this option are not complementary.

Option E - INCORRECT.
There's a missing part here, more than likely is different from more likely. One is more complete in terms of construction while the other isn't.