Find the value of 'a ' so that the function f(x) is defined by $f(x) = \begin{cases} \frac{\sin ^2 a x}{x^2} & x \neq 0 \\ 1 , & x =0 \end{cases}$ may be continuous at x = 0

Question

$(A)\; a =\pm1 $
$(B)\; a =0 $
$(C)\; a =-1 $
$(D)\; a =1 $