Answer:
c. negation
Explanation:
A conjunction is two statements joined by "and"; that does not fit this statement, so it is not a conjunction.
A disjunction is two statements joined by "or"; that does not fit this statement, so it is not a disjunction.
A conditional is an "if-then" statement; that does not fit this statement, so it is not a conditional.
This is a negation; the word "not" negates "the glass is always half full."