The correct option is A incalculable
Defection is the desertion of one's country or cause in favour of an opposing one. A citizen abandoning one's own country does not want to vote for it. The damage thus caused to democracy is too great to be estimated, that is, incalculable. Hence, Option A is correct. Among the rest of the options, inalienable means not subject to being taken away from or given away by the possessor, intolerable means unable to be endured, infallible means incapable of making mistakes or being wrong, indispensable means absolutely necessary. These do not express the intended meaning, hence incorrect.