💡 Detailed Explanation
The correct choice is 'まれ' (passive form indicator for Group 1 verbs). The verb '踏む' (fumu - to step on) conjugates to its passive form '踏まれる' (fumareru), yielding '踏まれて' in the te-form. This is an adversity passive showing the speaker was negatively affected. Sentence translation: 'Inside the crowded train this morning, someone stepped on my foot, and it was very painful.' 'ませ' / 'させ' indicate causatives (making someone step). 'られ' is the passive indicator for Group 2 verbs, whereas '踏む' is a Group 1 verb.