In this question, we are asked to identify which of the given options is a vector. A vector is a quantity that has both magnitude and direction. Let`s analyze each option to determine if it is a vector.

Option 1: Kinetic energy. Kinetic energy is a scalar quantity because it only has magnitude and does not have a specific direction associated with it. Therefore, option 1 is not a vector.

Option 2: Pressure. Pressure is also a scalar quantity. It is defined as force per unit area and does not have a direction associated with it. Therefore, option 2 is not a vector.

Option 3: Rate of change of momentum. The rate of change of momentum, also known as force, is a vector quantity. It has both magnitude and direction, as it is dependent on the magnitude and direction of the applied force. Therefore, option 3 is the correct answer as it is a vector.

Option 4: Density. Density is a scalar quantity that represents mass per unit volume. It does not have a specific direction associated with it. Therefore, option 4 is not a vector.

To summarize, only option 3, the rate of change of momentum, is a vector.