The correct option is C The weather being hot, we cancelled the trip.
Here, we have to combine the sentences by using nominative absolute construction.
A nominative absolute is a noun phrase. Most nominative absolutes contains a participle that modifies the noun and pronoun.
Option A is incorrect as 'so' is used to combine the sentences instead of nominative absolute construction.
Option B is incorrect as 'because' is used to join the sentences.
Option C is correct as 'being' is used to combine the sentences if there is an adjective (hot) after be-verb (was).
Option D is incorrect as 'since' is used to combine the sentences.