Of all the events leading to the formation of the Earth, the catalyst

IMO B
Structure of sentence:- of all events...., the catalyst is....
Clearly we will need superlative form for comparison i.e. most
A & D are out
E structure is not logical, hence, out.
Between B & C, to determine is the correct idiom here.

Based on below intended meaning, Eliminate A, C, and E

Determination vs Determine
Noun: Determination - the process of establishing something exactly by calculation or research
Verb: Determine - cause (something) to occur in a particular way or to have a particular nature

"Probably" to say that something has a high chance of happening - 50% or greater.
"Possibly" to say that something has a low chance of happening - 50% or less.
"Maybe" or "Perhaps" to say that something has an equal chance of happening or not happening

"Events leading to the formation of the Earth" is a process/action, in this case we need "verb", to express it, not the noun

Between B, and D
More is used when referring to a greater or additional amount or degree.
Most is used when referring to greatest in amount or degree.
The key difference between more and most is that while
More is considered as a comparative form,
Most is considered the superlative form.

(B) is probably the most difficult to determine - makes option B, our correct choice
(D) is probably the more difficult to determine