Refer to figures.
Air mass classification is based on the source region where the air mass forms and acquires its temperature and moisture characteristics. 

Polar maritime (Pm) air originates over cold ocean surfaces in polar or sub-polar latitudes. At its source, the air is in contact with a cold sea surface, which cools the lower layers and produces stable conditions with a negative or near-neutral lapse rate. The cold surface does not supply sufficient heat to generate convection, so the air mass is stable at source with stratiform cloud if any. However when polar maritime air moves toward lower latitudes over progressively warmer sea and land surfaces, it is heated from below, destabilised, and develops convective showers and good visibility. Polar maritime air is also characterised by high moisture content (maritime = ocean source) and low temperatures, distinguishing it from polar continental air which is cold and dry.

Stable conditions at its source → CORRECT. Polar maritime air originates over cold polar ocean surfaces where the cold sea cools the lower air layers and produces stable stratified conditions. The air is cold, moist, and stable at its source. As it travels toward lower latitudes it is heated from below and becomes progressively more unstable.

High temperatures and low relative humidity → INCORRECT. High temperatures and low relative humidity are characteristics of tropical continental (Tc) air, which originates over hot dry continental interiors. Polar maritime air is cold and moist, the opposite of this description.

Extreme instability and high moisture content at its source → INCORRECT. High moisture content is correct for maritime air but extreme instability at source is not. Polar maritime air is STABLE at source because the cold surface suppresses convection. Instability develops later as the air mass moves over warmer surfaces.

A high-pressure core that prevents any cloud formation → INCORRECT. Polar maritime source regions do not inherently have a high-pressure core that suppresses cloud. The stability at source may limit convective cloud but does not prevent all cloud formation. Stratiform and orographic cloud can develop in polar maritime air even at source.