Step 1: Combining drugs into one tablet (an FDC) is judged by the practical gains it delivers to therapy.
Step 2: Rationally chosen combinations can act synergistically to raise efficacy, so option a holds true.
Step 3: They can also lower adverse effects compared with high single-drug doses, and shrinking the number of tablets a patient must take boosts adherence, so options b and c are both valid.
Step 4: Because each listed advantage is genuine (with the bonus of reduced cost), the inclusive answer all of the above is correct.
\[\boxed{\text{All of the above}}\]