Q: That pollution harms the environment is self-evident;
that immediate government action to combat it is merited by the situation is a matter of debate.
(A) that immediate government action to combat it is merited by the situation – passive construction- choose active if available
(B) if the situation merits immediate action by the government – clause starts with ‘if..’ but is missing a ‘then’ to complete the sentence
(C) whether to combat the situation is merited by immediate government action – missing ‘or not’ (whether…or…)
(D)
that the situation merits immediate government action to combat it –
complete sentence after the colon(E) whether the government should immediately combat it is merited by the situation – seems to alter the meaning; ‘is merited by the situation’ followed by ‘is a matter of debate’ makes no sense.