Which of the following properties of a mass-on-a-spring system undergoing simple harmonic motion can be changed without affecting the frequency of oscillation?
amplitude
spring stillness
oscillating mass

If we are talking about simple harmonic motion, we are talking about waves and in this case the frequency is related to the period of oscillation in an inverse proportion.

Now, for a mass-on-a-spring system the period is given by:

Where is the oscillating mass and the spring constant, which depends on the spring stillness.

As we can see in this equation:

If we change and we will affect the period, hence the frequency.

Nevertheless, we do not see any relation with the Amplitude, this means the period (hence the frequency) does not depend on the amplitude.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.

a sonic boom is a loud boom that breaks the sound barrier

explanation: