Refer to figure.
The best technique for this type of question is to solve the easy part first, this will immediately discount two of the possible answers!

A compass is marked in degrees from 000 to 359 in a clockwise direction starting from North . Therefore, regardless of your heading, any turn to the left will result in a decrease in your heading in degrees.

For example if your heading is 180 degrees and you start turning left your heading indication will decrease through 170, 160, 150 etc. Therefore, it is normal for a left turn to show a decreasing heading value on the DRMC.

The DRMC, although simple and prone to errors, is fitted to all aircraft and commonly used as a standby instrument. It’s therefore very important that its indications are checked before you get airborne. The best way to do this is to check its indications with a known value such as the runway heading or another navigation system.

The question states that the DRMC indicates 20 degrees differently from the runway heading. Easy Access Rules for Large Aeroplanes (CS-25) states that the DRMC is allowed a maximum error of 10 degrees in normal level flight!

Therefore, even allowing for the fact that RW27 could have a magnetic Hdg of 275, the DRMC is unserviceable