The wavelength of light absorbed by $P_r$ form of phytochrome is

Phytochrome is the pigment that controls photoperiodic responses in plants. It exists in two interconvertible forms, $P_r$ and $P_{fr}$.
The $P_r$ form absorbs red light between 660 and 680 nm, with a peak at about 666 nm. It is the form synthesised in dark-grown seedlings.
When $P_r$ absorbs red light, it is converted into the $P_{fr}$ form.
The $P_{fr}$ form absorbs far-red light of about 730 nm and is converted back into $P_r$; it also reverts slowly in darkness.
$P_{fr}$ is the physiologically active form.
Among the given values, 680 nm lies in the red region absorbed by $P_r$, while 720 nm is in the far-red region absorbed by $P_{fr}$.
So the wavelength absorbed by the $P_r$ form is 680 nm.