To solve this problem, we need to identify which structure is homologous to the wing of a bird. Homologous structures are those that have a common evolutionary origin, even if their functions may differ.$\newline$Let's analyze each option:$\newline \bullet$Option 1: Dorsal fin of a shark - The dorsal fin of a shark is not homologous to the wing of a bird. Sharks are cartilaginous fish, and their fins are not derived from the same evolutionary origin as bird wings.$\newline \bullet$Option 2: Wing of a moth - The wing of a moth is analogous, not homologous, to the wing of a bird. Both are used for flying, but they evolved independently in insects and birds.$\newline \bullet$Option 3: Hind limb of a rabbit - The hind limb of a rabbit is homologous to the wing of a bird. Both are derived from the forelimbs of a common tetrapod ancestor, even though they serve different functions (locomotion vs. flight).$\newline \bullet$Option 4: Flipper of a whale - The flipper of a whale is homologous to the wing of a bird. Both are modified forelimbs that share a common evolutionary origin, adapted for different environments (swimming vs. flying).$\newline$Therefore, the correct answer is Option 4: Flipper of a whale.