Which mechanical device increases the pressure and density of a gas?

The appropriate mechanical device that increases the pressure and density of a gas is a compressor. Compressors work by mechanically reducing the volume of gas, which in turn raises its pressure and density. This process is critical in various applications, particularly in the transportation and storage of gases like compressed natural gas (CNG).

In contrast, a pump is primarily used to move liquids, not gases, and it operates by creating a difference in pressure to facilitate the flow of liquids rather than compressing them. A regulator serves a different function; it stabilizes and maintains the pressure level, ensuring it stays within a designated range but does not actively increase it. A vaporizer's role is also distinct; it converts liquids into gas, focusing on changing the state rather than increasing the pressure or density of gas.

Therefore, the compressor is the device specifically designed for increasing both the pressure and density of gaseous substances, making it the correct answer.