Areduction in the number of common cold viruses and other upper respiratory infections appears to be connected to exercising at least days a week.
a. two
b. three
c. four
d. five

A reduction in the number of common cold viruses and other upper respiratory infections appears to be connected to exercising at least (B) three days a week. Common cold is harmless but not to the point of having sore throat and cough plus sneezing and congestion. Then, you need to take care of your health.

A reduction in the number of common cold viruses and other upper respiratory infections appears to be connected to exercising at least three days a week - b.

Exercising three days a week has indeed in the past been the optimal recommendation for working out as it is the frequency when drastic benefits are being recorded. 

Awould be the most dangerous