According to the diagram conifers are
a. non-vascular spore-forming plants.
b. non-vascular seed-forming plants.
c. vascular spore-forming plants.
d. vascular seed-forming plants.

The correct answer is option D - vascular seed-forming plants.

Explanation:

The diagram explains that different types of plants are grouped on the basis of the common characters they share. For example, conifers are grouped together with cycads, ginkgo, gametophytes, and flowering plants as they share common which is seed-bearing plants.

Conifers and other seed plants also the group with other plants as ferns, horsetails, lycopods as they all have vascular tissues in the plants.

Thus, the correct answer is option - D. vascular seed-forming plants.

it is either c or d and i know for sure it is not a or b