The oppressive weather made the work difficult for them.

Answer: Option C

Turbulent : characterized by conflict, disorder, or confusion; not stable or calm.
Disappointing : failing to fulfil someone's hopes or expectations.
Unbearable : not able to be endured or tolerated.
Cold : of or at a low or relatively low temperature, especially when compared with the human body.
Oppressive : weighing heavily on the mind or spirits.

Synonym of Oppressive is Unbearable