Which metal would cause the greatest increase in the temperature of the water in the calorimeter: the one with the highest specific heat, or the one with the lower specific heat?

Answer;

the one with the highest specific heat

Explanation;The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.The specific heat of water is 1 calorie/gram °C or 4.186 joule/gram °C which is higher than any other common substance. A metal with the highest specific heat will cause the greatest increase in temperature of water in a calorimeter because the metal would hold more heat, and then transfer the greater quantity of heat to the water.

As newton's first law states a object in motion stays in motion

Gas will most likely  fill the container