The biological clock measures the length at each night by the relative amount of red absorbing and far-red absorbing phytochrome present at dawn, the amount of far-red absorbing phytochrome at dusk, the rate at which are kind of phytochrome is converted to the other, and the relative amount of red absorbing and far-red absorbing phytochrome at mid-day.

Phytochromes are a class of pigments that control most photomorphogenic responses in higher plants. Important plant responses that are regulated by the phytochrome system include photoperiodic induction of flowering, chloroplast development, leaf senescence and leaf abscission. Photoreceptors and circadian clocks (biological clocks) are universal mechanisms for sensing and responding to the light environment. Exposure to red light converts the chromoprotein to the functionally active form, while darkness or exposure to far-red light converts the chromophore to the inactive form. Plants grow toward sunlight because the red light from the sun converts the chromoprotein into the active form which triggers the plant growth. If seeds sense the light using the phytochrome centre they will germinate. So, the correct answer is option A.