In Relation to Discount Terms, What Does 5/7, N/30 Mean

In relation to discount terms, what does 5/7, n/30 mean?

A) Invoice due on 5^th July or 30^th November.
B) 50% discount if paid with 7 days, otherwise 30% discount.
C) 50% discount, November 30th
D) 5% discount to be deducted from the invoice price if payment is made within 7 days or the full amount is due within 30 days of the invoice date.