Constantine was the first roman emperor to embrace christianity (1), and he issued an edict that protected christians (2). this officially ended the persecution of christians and granted them the freedom to worship (3). but the edict failed and christians were persecuted until catholic leaders met at the council of trent (4).

The answer is 2.

Explanation:

Together with co-emperor Licinius, Constantine issued the edict of Milan in 313 AD that proclaimed tolerance of all religions in the empire. That meant stopping the persecution of Christians that was so ruthless under Diocletian, who had also divided the empire into two parts. Constantine´s  mother Helena converted into Christianity (St Helena) and was very strong in her commitment to the faith.

letter d is correct.

the answer is a, salt plain