Mass oscillates in mechanical waves like pressure waves, or sound waves caused due to vibration. There is no mass oscillating in an electromagnetic wave. Amplitude is a fixed quantity for a constant wave of a frequency. The amplitude could oscillate (vary periodically) if there are multiple frequencies in a wave. Frequency can also oscillate periodically if the wave is modulated. For example, in case of FM radio waves, the frequency oscillates in the air waves. In a simple single frequency wave with a constant amplitude, the quantity that varies as a Sine or cosine is the energy in the wave at a point or the energy that is transmitted along the direction of propagation is an expression with Sine function in it. The time average of energy is a constant. So the answer is energy, which is more appropriate than other choices given.