help!! a problem from prep!

Some patients who do not respond to the therapies for depression may simply have received inadequate treatment, having, for example, been prescribed a drug at a dosage too low to be effective or having been taken off a drug too soon.

a. having, for example, been prescribed a drug at a dosage too low to be effectivve or having been
    

b. having, for example, a drug prescription that was ineffectve because the dosage was too low, or being

c. as, for example, having too low of a dosage of a presdribed drug for it  to be effective, or being

d. when they have, for example, been prescribed too low a drug dosage to be effective, or were

e. for exanple, when they have a drug prescription with a dosage too low to be effective, or been

the answer is a.

can anybody tell me why?

Some patients may have received, having been ...,or having been...

"having been" here is to emphasize and modify what the patients did.

This is a perfect structure.