Which postulate of relativity was supported by the experiments of Michelson and Morley

Constancy of speed of light.

All observers measure the same speed of light,

no matter how any observer is moving.

The question is kind of awkward, though. It seems to say that

people were having a hard time accepting this postulate of relativity,

until Michelson and Morley came along and did their experiment, and

their results gave Relativity a boost.

Actually, Michelson and Morley did their famous experiment in 1887.

At the time, Einstein was only 8 years old, and Relativity was not even

a twinkle in his eye yet.