Question

Most primitive living vascular plants are
A. Sphagnum
B. Cycads
C. Brown algae
D. Ferns

Answer 1

Hint: Pteridophytes are known as the primitive vascular plants. In contrast to other vascular plants, the life cycle of Pteridophytes occurs in two phases, the gametophytic phase which is free living and the sporophytic phase. The gametophytic phase usually comprises archegonium (egg-producing) and antheridium (sperm-producing).

Complete answer:
Pteridophytes are also known as the ferns. Their plant body is well differentiated and these are primitive vascular plants which produce no flowers or seeds. On the other hand, Cycads are gymnosperms plants. Gymnosperms are vascular plants having a well differentiated plant body. They form naked seeds. Moreover, Sphagnum which is commonly known as peat moss belongs to the group bryophyta. Bryophytes consist of a well-differentiated plant body having defined stem and leaf-like structures but they lack specialized mechanical or vascular tissues. However, brown algae belongs to the group thallophyta. They do not have vascular tissues and their body is thalloid that is, it is not well differentiated into roots, stems and leaves.
Thus, the right option is D.

Note: A pteridophyte consists of conducting tissues say, xylem and phloem therefore these are also known as vascular plants. These plants are known to produce spores thereby developing into sporangia. These plants can be homosporous or heterosporous. Also, pteridophytes do not produce seeds or flowers therefore they are also referred to as "cryptogams", which implies that they have a hidden mode of reproduction.