# A response to length of day in plants is called

$\begin{array}{1 1}(A)\;\text{Phototropism}\\(B)\;\text{Photorespiration}\\(C)\;\text{Photoperiodism}\\(D)\;\text{Photo-oxidation}\end{array}$

A response to length of day in plants is called photoperiodism.
Hence (C) is the correct answer.