Answer “Magnetic variation information is stored in each IRS memory; it is applied to the true heading calculated by the respective IRS” is not generally correct but it is better than the other options.

There is no input from the gyro compass detector (flux valve) so “The main directional gyro which is coupled to the magnetic sensor (flux valve) positioned in the wingtip” is wrong.  

The FMS calculates magnetic heading and track from true heading and track, not from position, so “The FMS calculates MH and MT from the FMC position” is wrong.  

Magnetic variation is calculated by each IRS based on the respective IRS position and the aircraft true heading, not magnetic heading, so “Magnetic variation is calculated by each IRS based on the respective IRS position and the aircraft magnetic heading” is wrong.  

We are left with only one option!