The flowers which do not open at all are known as:

The correct answer is Option (2) → Cleistogamous flowers

Some plants such as Viola, Oxalis and Commelina make two kinds of flowers. One kind opens like normal flowers. The other kind never opens, so its anthers shed pollen straight onto the stigma inside the closed bud.

Option (1) Chasmogamous flowers — Incorrect. They are open flowers with exposed anthers and stigma, like those of most plants.

Option (2) Cleistogamous flowers — Correct. They do not open at all, so only self-pollination (autogamy) happens.

Option (3) Submerged flowers — Incorrect. These are flowers of water plants such as Zostera that stay under water; the term does not mean closed flowers.

Option (4) Cross-pollinated flowers — Incorrect. Cross-pollination needs open flowers, so a flower that never opens cannot be cross-pollinated.

NCERT Extract:
Pollination – “… Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all (Figure 1.9c).”