Which of the following pairs of structures is homologous?

Options

A. Wings of grasshopper and forelimbs of flying squirrel
B. Tentacles of Hydra and arms of starfish
C. Forelimbs of a bat and forelegs of a horse
D. Wings of a bird and wings of a moth

Answer

To determine which pair of structures is homologous, we need to understand the difference between homologous and analogous structures.

### Step-by-Step Solution:

1. **Understanding Homologous Structures**:
- Homologous structures are those that share a common ancestry but may serve different functions in the organisms. They indicate evolutionary relationships.

2. **Analyzing the Options**:
- **Option 1**: Wings of a grasshopper and forelimbs of a flying squirrel.
- These structures have different origins (insect vs. mammal) and serve similar functions (flight), making them analogous, not homologous.

- **Option 2**: Tentacles of hydra and arms of a starfish.
- These structures also have different origins and serve different functions, so they are not homologous.

- **Option 3**: Forelimbs of a bat and four legs of a horse.
- These structures share a common ancestry (both are derived from the same limb structure in a common ancestor) but serve different functions (flight vs. locomotion), making them homologous.

- **Option 4**: Wings of a bird and wings of a moth.
- These structures have different origins (bird is a vertebrate, moth is an insect) but serve the same function (flight), making them analogous.

3. **Conclusion**:
- The correct answer is **Option 3**: Forelimbs of a bat and four legs of a horse are homologous structures.