the taxi I stopped
a. とまった タクシー
b. とめた タクシー

I stopped the taxi, so it must be the choice with a transitive verb and that is b. とめたタクシー.

とまったタクシー means “the taxi which stopped (without your involvement).”

This question was taken from EasyJapaneseE’s“Transitive verbs vs Intransitive verbs” module. If you need to revise the Transitive verbs vs Intransitive verbs, this module will give you plenty of practice. For details, click the relevant link below.