Right answer:
estas
The demonstrative pronouns, such as este, ese and aquel can act as demonstrative adjectives when placed before a noun. So these adjectives demonstrate a quality about the noun they modify. Here that quality is the location in respect to the speaker or the listener. Estas match feminine plural noun just as chicas (girls) and comes from este being used to point out something near to the speaker in space or time. Finally:
Estas chicas son alumnas