Stem distinctly differentiated into node and internode in (a) Psilopsida (b) Lycopsida (c) Sphenopsida (d) Pteropsida
Hint: These plants are forming a major component of the coal-swamp vegetation. This plant belongs to the class Pteridophyta which appeared first in the Devonian and reached the peak of its abundance and diversity during the Carboniferous.
Complete answer: In sphenopsida the stem is distinctly differentiated into node and internode. This plant is also known as horsetails. Jointed stems mean stems with nodes and internodes along with whorls of leaves and branches that are born at the joints or nodes is the main characteristic of sphenopsida. The stem is vertically ridged at the internodal part of the plant. The sphenopsida consists of upright aerial stems that exhibit a monopodial branching pattern which has one main axis of growth.
Additional Information: -Usually at the tips of the fertile shoots the rings of sporangia are arranged in cones, where spores are produced. -Upright aerial stems are present in sphenopsida, which arise from a very extensive underground rhizome system. -Equisetum is the only living genus of sphenopsida is a small plant ranging between 4 or 5 cm and 12 m. -Calamites are the best-known fossil genera of sphenopsida that includes tree-like forms which grow up to 30m in height. -To the subdivision of Tracheophyta the psilopsida belongs and it is a vascular plant with no roots, leaves only partially differentiated or lacking, no leaf traces. -In Psilopsida a usually protostelic vascular cylinder, and therefore the sporangia merely terminal enlargements of the stem is found. -The Lycopsida reproduce by means of spores which are borne in sporangia on the edges of the stems at the bases of the leaves. -In several characters, pteropsida is different from lycopsida and sphenopsida.
So, the correct answer is, 'Sphenopsida'.
Note: -Another common fossil of sphenopsida is sphenophyllum. It was a slender plant with a ribbed stem that was only 1–7 mm in diameter but up to over several metres in length, that probably scrambled other vegetation. -Pteropsida are also called water fern. -Lycopsida bears branching stems with simple leaves known as microphylls.