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

Cleistogamous flowers

The correct answer is Option (2) → Cleistogamous flowers

Cleistogamous flower : Cleistogamous flowers are those that self-pollinate without opening. 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.

Cleistogamy refers to a reproductive strategy in plants where self-pollination occurs within closed flowers that do not open.  One advantage of this mechanism is that it eliminates the need for external pollinators such as insects or wind for successful pollination and fertilization. The flowers are capable of self-fertilization within their closed structure, allowing plants to reproduce even in the absence of pollinating agents.

In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.