The correct answer is: C. cross-magnetises it.
When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field is out of phase with the main field. This causes the armature field to cross-magnetise the main field, which reduces the overall flux in the machine. This reduction in flux causes the generator to produce less power.
Option A is incorrect because the armature field does not augment the main field directly. The armature field is out of phase with the main field, so it does not add directly to the main field.
Option B is incorrect because the armature field does not directly oppose the main field. The armature field is out of phase with the main field, so it does not subtract directly from the main field.
Option D is incorrect because the armature field does affect the main field. The armature field cross-magnetises the main field, which reduces the overall flux in the machine.