Assertion
In water, orthoboric acid behaves as a weak monobasic acid.

Reason
In water, orthoboric acid acts as a proton donor.

The correct option is C Assertion is correct but reason is incorrect.
Though orthoboric acid $\left(H_{3}B{O}_3\right)$ contains three $OH$ bonds but cannot ionize in water to give a proton $\left(H^{+}\right).$ Due to vacant orbitals, it accepts lone pair of electrons and acts as a Lewis acid. By taking one $O{H}^{-}$, it forms $B(OH{)}_4^{-}$ Hence, option (c) is the answer.