Step 1: The sentence implies a moral or ethical reason for not supporting the proposal. The word required should relate to inner moral sense.
Step 2: Option (A) conscious means being awake or aware, which does not fit the context.
Step 3: Option (B) consensus means general agreement, which is collective, not individual, hence incorrect.
Step 4: Option (C) conscience means moral sense of right and wrong, which perfectly fits the sentence.
Step 5: Option (D) consent means permission, which is not applicable here.
Thus, the correct word is conscience.