Option A - subtract the 3.5 degree from your azimuth.
To convert grid azimuth to magnetic azimuth you have to subtract the G - M angle from the grid azimuth. For example, if GM angle is 3.5 and grid azimuth is 150, magnetic azimuth will be
Magnetic azimuth = 150 - 3.5
Likewise, If you need to convert magnetic azimuth to grid azimuth, you will have to add magnetic azimuth to GM angle.