Hola Remy

The general rule is that adjectives that end in -o in their masculine form, e.g. guapo (handsome) take an -a in their feminine form: guapa

There is also a general rule that says that adjectives ending in -e or -a in their masculine singular form keep the same form in their feminine form, e.g. árbol verde, casa verde or chico deportista, chica deportista. Here the adjective is the same for both. 

In the question testing for the adjective "antiguo, -a" and all the set of questions related to regular adjectives (-o, -a) we've used basic adjectives, assuming students of that level are familiar with them and know what their masculine singular form is - so from that starting point you can work out how to form the others. 

I hope this clarified it.

Saludos

Inma