What is the most reliable measurement for the quantity of matter? volume density mass weight

Answer is: mass.

Mass is a fundamental measure of the amount of matter in the object.  Kilogram (symbol: kg) is the base unit of mass in the International System of Units.  

The weight is the force of gravity on the matter.  It is affected by gravity.

Volume is the amount of space that a substance occupies.  It is affected by temperature.

Density is a measure of mass per unit of volume (for example grams per milliliter). It is affected by temperature.

organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.