Answer:
Hi !!
A, B and D are correct....
C is not correct
At first, I was unsure about going to the Gym, then I went for a few days and found out I loved it !
Then = relates to time . It's an adverb. You can change it with afterwards (A,C) ,with "at that time" or as a consequence (D)
Than = most of the time it introduces a comparison (he is smaller than Paul)
"Rather than" is an adverb meaning "instead" (B)
hope I helped !!