Why is it important to understand the audience for your cover letter ?
a. so you can provide an explanation of why you left your previous jobs
b. so you can provide information that relates directly to the audiences needs
c. so you can provide references that your audience will be impressed by
d. so you can provide a short summary of your future goals aspirations

The answer is B: So you can provide information that relates directly to the audience´s needs.

A cover letter is a personalized document in which you address a specific audience in relation to a vacancy or an open job position, that is to say, it is usually addressed to a potential employer. It usually accompanies your CV, but unlike the CV, it is not a list of your professional and academic achievements, rather it is a composition of statements that relate to the audience´s specific needs and how they would benefit from employing you as well as how you can contribute further to their objectives.  

i will have to go with 1.

hope this !