Which of the following is an appropriate explanation of a zero-day attack?
a) In order to search for a vulnerable TCP/IP port that can be used as an entry point for
unauthorized intrusion, an attacker accesses a server while changing port numbers in
sequence.
b) An attacker obtains important security information, such as a password, from the
system administrator or user through skilled conversation or peeping, and then
accesses the system illegitimately by posing as the user.
c) When a vulnerability in software is identified, the identified vulnerability is
exploited to perform an attack before a patch for the software is provided by the
vendor.
d) In order to identify the password or crack the encrypted code, an attacker
successively attempts all character strings that are generated by mixing upper-case
characters and lower-case characters of a term from the dictionary, or adding
numeric characters to such a term.