The correct answer is A.
To rearrange the equation, you have to divide both sides by "m" to isolate "a" so that acceleration can be directly calculated. This would result in the equation becoming:
f/m = ma/m
As the mass (m) on the right hand side of the equation appears in the numerator and the denominator, it can be cancelled, resulting in the new equation:
f/m = a
which can then be flipped to give a = f/m.
