Question

Preencha o espaço com a forma superlativa ou comparativa do adjetivo entre parênteses.
Which dress do you think is _______ : the black one or the purple one? (pretty)

Answer 1

olá,
Prettier , poIs temos que conjugar o verbo

Answer 2

which dress do you think is the prettiest? (means which one is the best amoung all of them.)

whereas:

which dress do you think is the prettier? (sound like choose between couple things, like A or B.) 

Im not certain at all, but both sounds ok, it depends on the context.