Pressure altitude = An atmospheric pressure expressed in terms of altitude which corresponds to that pressure in the Standard Atmosphere.

The question asks about the conditions which lower an aircraft’s true altitude compared to pressure altitude.

We know the following about pressure and temperature:

• When an aircraft is in a lower pressure area, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a higher-pressure area, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
• When an aircraft is in a colder airmass than ISA, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a warmer airmass than ISA, the altimeter will under read. The aircrafts true altitude will be higher than indicated.

In cold temperatures and low-pressure conditions, the density will be increased, because of this the pressure levels would be more closely spaced together and thus give an indication of being higher than actual.

That is why a flight over mountains would be most dangerous because the true altitude of your aircraft will be lower than the altimeter indicates. You will have less safety margin between the aircraft and the mountain.