The ceteris paribus assumption is employed in economic analysis, in order to:

Let's start with the meaning of ceteris paribus , it is a Latin phrase which means keeping all the other things constant or unchanged . 
Now options were
 A)Focus on the effect of a single factor on a certain variable
 B)Make sure that all relevant factors are considered
 C)Distinguish macroeconomics from microeconomics
 D)Avoid making normative statements
 So from above definition we can understand that it works only on one
variable keeping others constant.
 So the answer will be
 A)Focus on the effect of a single factor on a certain variable

The answer should be b.