(A) the primary energy source in many regions.

Solution

Among the given options, starters (i) and (ii) can be used to combine the given statements in a grammatically and contextually meaningful way. The compositions thus formed would be: (i) Although renewable energy sources are becoming more affordable, fossil fuels are still widely used as the primary energy source in many regions. (ii) Even though renewable energy sources are becoming more affordable, fossil fuels are still widely used as the primary energy source in many regions. The starter "foremost" is an adjective that means "most prominent in rank, importance, or position" and does not fit the context of the sentence as a conjunction or a clause introducer. It does not help in forming a coherent and grammatically correct composition when combining these three statements.