Most bryophytes require a damp habitat because :

Options

A. they do not have a cuticle
B. they lack gametangia to protect their developing embryo
C. their sporophyte generation, which lacks vascular tissue, is dependent on the gametophyte
D. flagellated sperm must swim to fertilize the egg

Explanation

### Step-by-Step Solution:

1. **Understanding Bryophytes**: Begin by recognizing that bryophytes are non-vascular plants, which include mosses, liverworts, and hornworts. They are often referred to as the amphibians of the plant kingdom because they thrive in moist environments.

2. **Reproductive Process**: Bryophytes reproduce via a life cycle that includes both gametophyte and sporophyte generations. The male gametes (sperm) are flagellated, meaning they can swim, while the female gametes (eggs) are non-motile.

3. **Role of Water**: For fertilization to occur, the flagellated sperm must swim through water to reach the non-motile egg. This necessitates a damp habitat, as without water, the sperm cannot move to fertilize the egg.

4. **Evaluating the Options**:
- **Option A**: They do not have a cuticle. This is incorrect because the presence or absence of a cuticle does not directly relate to the need for a damp habitat.
- **Option B**: They lack gametangia to protect their developing embryo. This is also incorrect; bryophytes do have gametangia.
- **Option C**: Their sporophyte generation, which lacks vascular tissue, is dependent on the gametophyte. While this is true, it does not explain the need for a damp habitat.
- **Option D**: Flagellated sperm must swim to fertilize the eggs. This is the correct answer, as it directly relates to the necessity of water for reproduction.

5. **Conclusion**: Based on the evaluation of the options, the correct answer is **Option D**: Flagellated sperm must swim to fertilize the eggs.