The correct answer is (a).
“Not proved” means that something has not been proven to be true. It does not mean that it is false, or that it has been disproved. It simply means that there is no evidence to support the claim that it is true.
(b) is incorrect because it implies that something has been proven to be false. This is not necessarily the case if something is not proved to be true.
(c) is incorrect because it implies that something is neither true nor false. This is not necessarily the case if something is not proved to be true. It is possible that something is true, but there is simply no evidence to support that claim.
(d) is incorrect because it implies that something is denied. This is not necessarily the case if something is not proved to be true. It is possible that something is true, but there is simply no evidence to support that claim.