The correct answer is Longitudinal. Explanation & Key Takeaways: • Longitudinal waves are mechanical waves where the particles of the medium move parallel to the direction of wave propagation. • When a piston moves back and forth in a liquid-filled cylinder, it compresses and rarefies the liquid, creating regions of high and low pressure. • These pressure variations result in the formation of longitudinal waves in the liquid. • Longitudinal waves require a medium to propagate, such as solids, liquids, or gases, as they depend on the medium's elasticity and density. Important Notes: • Longitudinal waves involve the transfer of energy through compressions (regions of high pressure) and rarefactions (regions of low pressure). • The speed of longitudinal waves depends on the properties of the medium, such as its density and bulk modulus. • Examples of longitudinal waves include sound waves in the air, seismic P-waves, and ultrasonic waves in liquids and solids. • The particles in the medium do not travel with the wave; they oscillate around their mean positions as energy is transferred through the medium.