Question

Her proposal was not acceptable.the proposal was against my nature.(join into complex sentence)

Answer 1

his proposal was not accepted as the proposal was against my nature ...
H O P E I T H E L P S U

MARK ME AS BRAINLIEST

AND FOLLOW ME

Answer 2

Her proposal was not acceptable since it was against my nature is the complex sentence for the above sentence.  

To make the above sentence into a complex sentence, you can also use the works like As, Because, etc.

A complex sentence is a sentence which will contain a subordinate clause or clauses, so the above answer is undoubtedly a correct one.