The in operator in Python can be used to check if a particular substring exists within a string. This is why it is the correct choice. The += operator is used to append a value to an existing variable, typically used in loops for concatenation or arithmetic operations but doesn't check the existence of a substring. The == operator compares two values for equality, which is not useful when looking for a substring within a larger string. % is the modulus operator in arithmetic operations and can be used as a string formatting operator in Python, but it does not check for substring existence either.