The terms on an invoice are $\frac{4}{15},\frac{n}{30}.$ This means that

a$.$ a discount of $4\%$ will be allowed if payment is made by the $15$ th of the month.

b$.$ a discount of $15\%$ will be allowed if payment is made by the $4$ th day after the date of the invoice.

c$.$ a discount of $4\%$ will be allowed if payment is made within $15$ days from the date of the invoice.

d$.$ a discount of $15\%$ will be allowed if payment is made with $4$ days from the date of the invoice.