The tour was cancelled on account of incessant rain.

Answer: Option A

Constant : occurring continuously over a period of time.
Heavy : of great weight; difficult to lift or move.
Intermittent : occurring at irregular intervals; not continuous or steady.
Unexpected : not expected or regarded as likely to happen.
Incessant : (of something regarded as unpleasant) continuing without pause or interruption.

Synonym of Incessant is Constant