A/an is a sudden worsening in the severity of the symptoms or signs.

Exacerbation is the term which is referred to a sudden worsening in the severity of the symptoms or signs.
For example, an asthma.
It may occur or worsen through different factors, like air pollution, smoke, dusts, etc. This is called exacerbation.

Your answer is going to be c. 

It is less likely that the shoe will slip while someone is walking