Question

what theorem is it using the choices ?

Answer 1

They are similar by the Side-Side-Side Theorem. 

The smaller triangle is 3/4 of the second triangle:
90/120; divide each by 10, = 9 / 12 which simplifies to 3/4

135/180; divide each by 15, = 9 / 12 which simplifies to 3/4

117/156; divide each by 13, = 9 / 12 which simplifies to 3/4

Answer 2

it is using the bottom