If $a \cos \theta- b \sin \theta =c$ then $(a \cos \theta)$=________

Question

$\begin{array}{1 1}(A) \; \sqrt{a^2+b^-c^2} \\(B)\; -\sqrt {a^2+b^2-c^2} \\(C)\;\pm \sqrt {a^2+b^2-c^2} \\(D)\;a^2+b^2-c^2 \end{array}$