The correct option is A that
A subordinate clause ("he was blinded by it") depends on the main clause ("The snow was so dazzlingly white") for its full meaning in a sentence. When the subordinate clause is the result or effect of the main clause "was so dazzlingly white", a subordinating pair of conjunctions 'so...that' is used. 'So' is already present in the sentence "so dazzlingly white", hence 'for' needs to be replaced by 'that', and option A is correct. 'For' is an Illative, coordinating conjunction. Illative conjunctions are used for those sentences where one statement or fact is provided by or inferred from the other. This type of conjunction is 'coordinating', i.e., when the statements in a sentence are of equal rank or importance, as they're independent on their own. The given sentence does not have independent clauses, as one of them is dependent on the other. The sentence requires a subordinating, and not a coordinating conjunction, hence option D is incorrect. 'Because' is used for cause-related sentences, where the subordinate clause is the cause or justification for the event or action in the main clause, whereas the clause in the sentence is the result, and not the cause. Hence option B is incorrect. 'But' is an adversative conjunction used when one clause is opposite or contrast in meaning to the other, in a sentence. As the given sentence is not contrast, the option is not appropriate, and option C is incorrect.