Answer:

Introduction:
In this question, we are asked to select the pair of important features that distinguish Gnetum from Cycas and Pinus and show affinities with angiosperms. Let's analyze each option and explain why option B is the correct answer.

Option A: Absence of resin duct and leaf venation:
- Gnetum: Gnetum possesses resin ducts, which are specialized structures that produce and store resin. Gnetum also has reticulate venation in its leaves.
- Cycas and Pinus: Both Cycas and Pinus have resin ducts and reticulate venation in their leaves.
- Affinities with angiosperms: The absence of resin ducts and leaf venation is not a distinguishing feature of Gnetum and does not show any particular affinity with angiosperms. This option is incorrect.

Option B: Presence of vessel elements and absence of archegonia:
- Gnetum: Gnetum possesses vessel elements in its xylem, which are specialized cells that allow for efficient water transport. Gnetum does not have archegonia, which are female reproductive structures found in some plants.
- Cycas and Pinus: Both Cycas and Pinus lack vessel elements in their xylem but have archegonia.
- Affinities with angiosperms: The presence of vessel elements and absence of archegonia in Gnetum are important features that show affinities with angiosperms. Vessel elements are a characteristic feature of angiosperms and are involved in their efficient water transport system. This option is correct.

Option C: Perianth and two integuments:
- Gnetum: Gnetum lacks a perianth, which is the outermost part of a flower. It also has a single integument.
- Cycas and Pinus: Both Cycas and Pinus have a perianth and two integuments.
- Affinities with angiosperms: The absence of a perianth and presence of a single integument in Gnetum do not show any particular affinity with angiosperms. This option is incorrect.

Option D: Embryo development and apical meristem:
- Gnetum: Gnetum has a unique embryo development pattern in which the suspensor elongates and pushes the embryo into the endosperm. It also possesses an apical meristem, which is a region of actively dividing cells at the tip of the stem or root.
- Cycas and Pinus: Both Cycas and Pinus have different embryo development patterns and possess apical meristems.
- Affinities with angiosperms: Although Gnetum has unique embryo development, this feature does not specifically show affinities with angiosperms. This option is incorrect.

Conclusion:
After analyzing each option, we can conclude that the correct answer is option B: Presence of vessel elements and absence of archegonia. This pair of features distinguishes Gnetum from Cycas and Pinus and shows affinities with angiosperms. Vessel elements are a characteristic feature of angiosperms and their presence in Gnetum indicates a closer relationship with angiosperms. The absence of archegonia in Gnetum further supports its affinity with angiosperms, as archeg