Final answer:
Betelgeuse is the star most likely to turn into a black hole first among Betelgeuse, Bellatrix, Polaris, and the Sun.
Step-by-step explanation:
Based on their mass, the star most likely to turn into a black hole first among Betelgeuse, Bellatrix, Polaris, and the Sun would be Betelgeuse.
This is because Betelgeuse is a red supergiant star with a mass estimated to be about 20 times that of the Sun, making it a prime candidate for a black hole formation.