Identify the option that most appropriately combines both the given sentences into one sentence.
He was adamant to undertake the journey to the hills. He had promised to visit his school.

Correct option is </B>A

The correct option is (a) because "for" effectively explains the reason behind his determination to undertake the journey. His promise to visit his school is the motivation for his decision to travel to the hills. Option (b) is grammatically incorrect, option (c) introduces an illogical condition, and option (d) uses "forever" incorrectly.