The correct option is C 3
Degree of a differential equation is defined as the highest power of highest derivative in it after differential equation is cleared of radicals and fractions so far as the derivatives are concerned. dydx is inside the radical symbol(cube root).
Cubing both the sides, (d2ydx2)3=1+(dydx)4
The degree will be power of d2ydx2 which is 3.