What percentage of the keyspace must an attacker generally search through until he or she finds the key that decrypts the data?

In a brute-force attack, what percentage of the keyspace must an attacker generally search through until he or she finds the key that decrypts the data?
A. Roughly 50 percent
B. Roughly 66 percent
C. Roughly 75 percent
D. Roughly 10 percent