Importance of day length in flowering of plants was first shown in

Which of the following exerts a profound effect on the reproductive growth of a flowering plant?

When flowering is regulated by the length of day and night,it is called

$A$ group of plants shows flowering in a $12$ hour day and $12$ hour night cycle. When the night phase of another group of the same plants is interrupted by a brief flash of light, flowering does not occur. In which of the following groups would you place these plants?

Florigen results in the conversion of

Phytochrome is closely related to