The best statement that evaluates why Constantine I converted to Christianity during his fight to gain control of the Roman Empire is:
"Constantine thought he saw a cross in the sky the night before a big battle and believed this celestial clue helped him win the battle."
This statement is best supported by historical accounts, particularly the story of the Battle of Milvian Bridge in 312 AD. According to these accounts, Constantine had a vision the night before the battle where he saw a cross of light in the sky along with the words "In this sign, conquer." He interpreted this as a divine message that prompted him to embrace Christianity, which he believed contributed to his victory against Maxentius. This experience was a pivotal moment in his life, influencing not just his personal beliefs, but also the future of Christianity in the Roman Empire.
The other statements either misrepresent the timeline of his conversion or inaccurately state his beliefs at the time of his death.