Correct Answer: Important event

Solution : The correct choice is the s econd option , important event .

A milestone is a significant event or stage in the development or progress of something. In this context, the sentence suggests that getting the long-awaited promotion is a significant achievement or turning point in the person's career.

The meanings of the other options are as follows:

• Burden refers to something that is difficult to bear, a heavy load, or a responsibility that weighs heavily on someone.
• Wonder can be both a noun and a verb. As a noun, it refers to a feeling of amazement, curiosity, or awe towards something remarkable.
• Mishap refers to an unfortunate or unlucky event, an accident, or a minor misfortune that causes inconvenience, trouble, or distress.