Answer
Check below for the answer.
Work Step by Step
In (a), the boiling destroys all living microorganisms in the broth and causes the air to be forced out along with any organisms in it.
In (b), as air returned to the flask, it contained microorganisms but these would be trapped by the curvature of the flask and hence won't reach the broth.
In (c), when the flask is tipped, the sterile broth comes into contact with the dust and hence gets contaminated.
