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Solution

Given Identity is:

cos(4x) cos(2x)+sin(4x) sin(2x)

=cos(4x-2x)

=cos(2x)

hence option (b) is correct.

because the angle sum formula is cos(A-B)= cos A cos B+ sin A sin B.

Therofore

Right hand side =cos(2x)

= cos((4-2)x)

=cos(4x-2x)

=cos(4x) cos(2x)+sin(4x) sin(2x)

=Left hand side