Question

which property determines a stars lifetime on the main sequence?
luminosity
composition
mass
temperature

Explanation:

A star's mass is the key factor. More massive stars have higher core temperatures and pressures. This leads to faster nuclear fusion (hydrogen - to - helium burning). Faster fusion means the star consumes its fuel (hydrogen) more quickly. Luminosity is related to mass (more massive stars are more luminous), but it is mass that fundamentally drives the rate of fusion. Composition (like the amount of hydrogen vs. other elements) has some influence, but mass is the dominant factor. Temperature is related to mass (via the mass - luminosity - temperature relationship in main - sequence stars), but again, mass is the primary determinant.

Answer:

Mass