Default risk is the risk that...

a$.$

$...$a bond issuer is unable to make a profit.

b$.$

$...$a bond issuer is unable to pay the face value at maturity.

c$.$

$...$a bond issuer is unable to pay the its phone bill.

d$.$

$...$a bond issuer is unable to make interest payments.

e$.$

$......$a bond issuer is unable to make a revenue.