When we have the boolean variables A and B and we want to analyze the disjuction A OR B, this sentence will be true if at least one of the two variables is true.
This way, the truth table is:
A B A OR B
T T T
T F T
F T T
F F F
(The sentence only is false when both variables are false).
In the same way, if we add a third variable C and we want to analyze A OR B OR C, the sentence only is false if all three variables are false.
Also, when constructing a truth table, the normal procedure is switching first the last variable, then going from the right to the left, so the first variable will be switched only one time:
A B C
T T T
T T F
T F T
T F F
F T T
F T F
F F T
F F F
Therefore the correct option is C.