Which of the following types of credits would best describe credit cards?
a. closed and secured
b. closed and unsecured
c. open and secured
d. open and unsecured

I believe the answer is: open and unsecured 

Open credit refers to a type of credit that provided money to be borrowed at a certain time when we need it. 
And unsecured credit refers to a type of credit that does not protected by asset collateral.
Credit cards have both of these characteristics.