Verified answer

Most pulsating variable stars inhabit a section of the H-R diagram called the instability strip. A special category of very luminous pulsating variables lies at the top of this strip: the Cepheid Variables, or Cepheids for short. These stars fluctuate in luminosity with periods of a few days to a few months. In 1912, Henrietta Leavitt, an astronomer at Harvard, discovered that the periods of these stars are very closely related to their luminosities. The longer the period, the more luminous the star. This Period-Luminosity Relation is seen to be true because larger (and hence more luminous) Cepheids take longer to pulsate back and forth in size.

So if two Cepheids have have the same period then they have the same brightness.

But if one appears dimmer it must be further away.

So galaxy B is further away than Galaxy A

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Answer B

And this question has been answered earlier.