The correct answer is A. Temperature.
Contact metamorphism is a type of metamorphism that occurs when hot magma or lava comes into contact with surrounding rocks. The heat from the magma or lava causes the surrounding rocks to recrystallize, or change their mineral composition. The degree of metamorphism depends on the temperature of the magma or lava and the length of time that the rocks are in contact with it.
Pressure is also a factor in contact metamorphism, but it is not as important as temperature. The pressure from the magma or lava can cause the surrounding rocks to deform, but it does not usually cause them to recrystallize.
Weathering is the process by which rocks are broken down by the elements, such as water, wind, and ice. Weathering does not cause rocks to recrystallize, but it can change their physical appearance.
Flowing water can also cause rocks to weather, but it is not a major factor in contact metamorphism.
In conclusion, the primary agent of contact metamorphism is temperature.