Choose the correct answer from the alternatives given.
Light of wavelengths causes phytochrome to switch from inactive to active form whereas light of wavelengths has the opposite effect.

Phytochrome is receptors of light which are a blue-green pigment and are dependent upon red and far-red light for the process of flowering. The phytochromes are present in two forms:

1. Active form (Pfr) 
2. Inactive form (Pr)

When the plants are exposed to a red light then phytochromes get activated (Pfr) and the help in the growth of the plant then later when exposed to a far-red light it gets deactivated and becomes inactive form (Pr) thus resulting in the stoppage of flowering.

So, the correct option is 'Red; far-red'.