Account a) is the correct as although both balance, you need to look at what each account represents and where the double entry will be. As you are dealing with purchases then credit purchases will make Masons Ltd a Creditor which should have a Cr. balance as a) does. Also, look at other items in the account, if you bought goods for resale from Masons Ltd you would:
Cr. Masons Ltd (PLCA/Creditors)
And if you were making a payment to Masons Ltd you would:
Dr. Masons Ltd (PLCA/Creditors)
This is all happening correctly in account a). Likewise the opposite can be said of account b).
I hope this makes sense.