Ferns have vascular tissue, an adaptation to life on land, but they have an ancestral reproductive trait that tends to limit them to moist habitats. What is it?

The correct answer is - flagellated sperm.

Explanation:

In ancestral trait that is flagellated sperm in ferns that were limited to moist habitat has sperms that must swim to the eggs. This is why ferns often require splashing water for the movement of sperm to points near female gametangia during reproduction.

Fern now adapted the vascular system to get life on the life and they are not limited to the water bodies, however, flagellated sperm is the ancestral reproductive trait.