In SQL, the correct usage of conversion functions is crucial when performing operations on dates. Oracle uses the TO_DATE function to convert a string to a date, and the TO_CHAR function to convert dates or numbers to strings.
Statement C is correct: WHERE order_date > TO_DATE('JUL 10 2018','MON DD YYYY'); is a proper use of the TO_DATE function. It converts the string 'JUL 10 2018' to a date type, with the format 'MON DD YYYY', which is then used to compare with the order_date.
Statements A, B, D, and E are incorrect or misuse conversion functions:
A is incorrect because TO_CHAR is used to convert dates or numbers to strings, not the other way around, and therefore should not be compared with order_date.
B is incorrect because order_date is of type DATE, and you should not compare a DATE with a string without converting it; the TO_CHAR here should be TO_DATE.
D is incorrect because it mixes TO_DATE and TO_CHAR in the same IN list, which should contain date types only.
E is incorrect because TO_DATE should take a string as an argument, not a date returned by ADD_MONTHS.

Explanation/Reference: https://blogs.oracle.com/opal/sqlplus-101-substitution-variables#4_1_8

Explanation
References:
http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_2012.htm#SQLRF00817
https://docs.oracle.com/cd/A84870_01/doc/server.816/a76989/ch26.htm