In prder to be presented in the simplest form, the algebraic expression must not have anything left to be solved. Let's analyze one by one:
a)

Answer: NOT in the simplest form
b)

Answer: NOT in the simplest form
c)

Answer: YES, it's in its simplest form, no similar terms to work in.
d)

Answer: YES, it's in its simplest form, no similar terms to work in.
e)

Answer: NOT in the simplest form
f)

Answer: YES, it's in its simplest form, no similar terms to work in.