Step 1: Standard definitions, allopatric means different areas, sympatric means the same area.
Step 2: This question's Assertion and Reason use the reverse pairing of these terms.
Step 3: Per the key, both are accepted as correct as worded, but R (about sympatric species) does not explain A (about allopatric species).
Step 4: Correct answer, Both (A) and (R) correct, but (R) is not the correct explanation of (A).