To solve this question, the user must know the basic structure of an If...Then statement.

An If...Then statement is a conditional statement that tests whether a certain condition is true or false. It has two main parts: the "if" clause, which tests the condition, and the "then" clause, which specifies what to do if the condition is true.

Now let's go through each option and explain why it is right or wrong:

A. If: This statement is not optional in an If...Then statement. It is the first part of the statement and is necessary to test the condition.

B. Then: This statement is not optional in an If...Then statement. It is the second part of the statement and is necessary to specify what to do if the condition is true.

C. Else: This statement is optional in an If...Then statement. It provides an alternative action to take if the condition is false. If a programmer does not want to specify an alternative action, they can omit the Else statement.

D. Both a and b: This option is incorrect because only statement c (Else) is optional, while statements a (If) and b (Then) are required.

E. All of the above: This option is incorrect. Only statement c (Else) is optional, while statements a (If) and b (Then) are required.

Therefore, the correct answer is: C. Else.