keep fit (3)

Can I say.

(a) They are doing exercise to become fit / to keep fit.

(b) Do more exercise can become fit.

Does exercise will become fit.

(c) They do exercise every day to keep their body fit.

(d) They exercise every day so they will be fit.

They do exercise to keep their body fit every day.

They exercise every day because they want to fit as a fiddle.