Gibberellin is a plant hormone which helps in growth of stem. The rapid growth and flowering is called as bolting. Bolting requires either long days or cold treatment. However, it can be induced artificially by the application of gibberellin under normal conditions. Hence, it replaces the long day conditions required for flowering. It also controls sex expression in many plant species. It promotes maleness in flower.

Control of flowering in long day plants and sex expression in certain species

The correct answer is option D - Gibberellins.

Gibberellins are a group of plant hormones that play a crucial role in controlling various aspects of plant growth and development, including flowering in long day plants and sex expression in certain species.

Gibberellins and Flowering in Long Day Plants:
- Long day plants are those plants that require a certain minimum photoperiod (duration of light) to flower.
- Gibberellins are involved in the regulation of flowering in long day plants by promoting the transition from vegetative growth to reproductive growth.
- In long day plants, flowering is induced when the photoperiod exceeds a certain critical length.
- Gibberellins are synthesized in the leaves and transported to the shoot apical meristem (SAM), where they promote the expression of genes involved in flower formation.
- The presence of gibberellins stimulates the elongation of the stem and promotes the development of floral buds, leading to flowering.

Gibberellins and Sex Expression in Certain Species:
- In some plant species, the sex expression of flowers is determined by the balance of gibberellins and other hormones.
- For example, in cucurbits (such as cucumbers, melons, and pumpkins), gibberellins control the sex expression of flowers.
- Female flowers are induced by a high ratio of gibberellins to auxins in the growing tips, while male flowers are induced by a low ratio of gibberellins to auxins.
- The application of exogenous gibberellins can influence the sex expression of flowers in these plants.
- By manipulating the gibberellin levels, it is possible to control the sex expression and increase the yield of desired fruits in cucurbits.

Conclusion:
Gibberellins are plant hormones that regulate various aspects of plant growth and development, including flowering in long day plants and sex expression in certain species. They play a key role in promoting flower formation in long day plants and influencing the sex expression of flowers in species like cucurbits.