I apologize for my delay in answering your inquiry. The problem was that I had to check with our suppliers to see whether we could provide the item in the quantity you say you want. We can.
a) I checked with our suppliers and they will be able to fill your order.
b) I am so relieved that your order can be filled.
c) We can provide the requested item in the quantity you desire.

We can provide the requested item in the quantity you desire

Explanation:

We can provide the requested item in the quantity you desire is the most suitable answer to this question.

but lets see why other option can not be correct answer

option a says that their supplier wont be able to deliver while customer is asking service from him, it should be answered from that perspective moreover positive answer leaves better impact on customer so long term relationship can be established.

option B cannot be correct answer as it says that protagonist is relieved which suggest some doubt about its firm capability to deliver the product.  

hence option c is correct answer as it starts on a optimistic note and there is explicit message about deliverable.