Edwin hubble was the first to suggest that the universe is expanding. we can verify his theory by observing the of distant stars and galaxies.

If the universe is expanding, this means that all the distances are getting bigger too- those differences are expanding too. 

This change in the difference between the galaxies can be measured by the redshift of distant stars and galaxies. -this is the correct answer. The Redshift was already discovered by Hubble. 

Hoped this helped happy studying