The larger triangle ABC is similar to the smaller triangles as follows.

Triangle ABC ~ triangle DBA by AA similarity because they share angle B and angles A and D are right angles.

Triangle ABC ~ triangle DAC by AA similarity because they share angle C and angles A and D are right angles.

So you can set up the following proportions, seeing that the answers are involving either AC^2 or AB^2.

AB/BD = BC/AB When you cross multiply, you get AB^2 = BC times BD, which is the first answer listed.

Using the other proportion, AC/CD = BC/AC, when you cross multiply, you get AC^2 = BC times CD, which is not one of the answers listed.