Final answer:
In an open universe, the expansion continues forever, leading to stars and galaxies becoming increasingly isolated. Over immense time scales, galaxies will effectively 'evaporate', and all matter will decay into photons and subatomic particles, resulting in a dark, cold universe.
Step-by-step explanation:
The ultimate fate of an open universe, according to current cosmological models, is one where the universe continues expanding indefinitely. The density of an open universe is less than the critical density, meaning gravity is never important enough to stop the expansion. In this scenario, contrasted with a closed universe which might experience a Big Crunch, stars and galaxies will drift further apart, and stellar evolution will continue within galaxies until stars cease to shine and become remnants such as white dwarfs, neutron stars, and black holes. Eventually, these too will cool or cease emitting energy, leading to a dark and cold universe.
In such a universe, time and space have no end, and it expands forever, always a bit more slowly due to the spreading out of matter and energy. As stars end their life cycles and galaxy lights go out, the universe will become a vast, cold expanse with isolated, dark galaxies drifting endlessly apart in a ceaseless expansion. Galaxies will effectively evaporate, and all matter will eventually decay into a low-density sea of photons and subatomic particles. Thus, the most accurate depiction for the ultimate fate of an open universe is option B, with elements of C also becoming true over vast time scales.