How would you write a C++ function to remove all occurrences of the letter 'a' from a given string?
a) remove_a(word) { /* code to remove 'a' / }
b) remove_a_string(word) { / code to remove 'a' / }
c) string remove_a(string word) { / code to remove 'a' / }
d) void remove_a(string word) { / code to remove 'a' */ }