The correct option is C neither
When a conjunction presents two alternatives in a sentence, that conjunction is called a disjunctive or alternative conjunction. Some of them are in pairs, like, "neither...nor", "either...or", "not...neither" and so on. The given sentence contains the negative word "not", which is paired up with "neither", and not "either". 'Either' is used for positive statements whereas the one given is negative, hence the underlined word is not appropriate and option D is incorrect. 'Neither' is used for a negative sentence with alternatives, and is paired with 'nor' or 'not', where the other is as "neither...nor" and "not...neither", hence option C is correct. 'Although' is a subordinating conjunction used for contrasting sentences, where the clauses in a sentence are of unequal rank or importance, one clause is dependent on the other for its meaning and is hence, subordinate. Since the given sentence is neither contrast in meaning, nor does it contain a subordinate clause, option A is incorrect. 'But' is an adversative, coordinating conjunction used when a sentence has two clauses of a contrast meaning, and the given sentence is not adversative, but alternative. Hence, option B is incorrect.