Hint:let us first understand what is meant by red light phytochrome. Phytochromes are a type of photoreceptor pigment in plants, fungi and bacteria that are used in the detection of light. In the visible spectrum, in red and far-red regions they are sensitive to light.
In plants that are subjected to continuous red light the inactive form of phytochrome Pi is converted into the active form of phytochrome Pfr. Due to the prolonged exposure of red light, there is a rapid destruction of phytochrome. When the phytochrome level goes below the critical value, there will be de novo synthesis of inactive phytochrome Pi. Hence the destruction and synthesis of phytochrome remains balanced.
Additional information: Recent studies have proposed that phytochromes also act as the temperature sensors. Warmer the temperature, greater the deactivation of phytochrome. They regulate many aspects of plants. They control the germination of seeds, manage the synthesis of chlorophyll and also the elongation of seedling. They also regulate the shape, size, number and movement of the leaves and the flower timing in adult plants. Along with phytochrome, some plants also include the phototropins and cryptochromes that respond to ultraviolet R8 and ultraviolet-A and blue light, which is sensitive to ultraviolet-B light.
Therefore the correct answer is option (D) Destruction and synthesis remain in balance.
Note:Unlike other pigments, Phytochrome exists in two forms of interconvertible chemical structures. They are named based on the color of the light that is absorbed maximum. A blue form is Pr that absorbs the light of wavelength 660nm which is red light. It is the original form of phytochrome. Pfr is a blue-green form that absorbs the light of wavelength 730 nm which is a far red light.