A conjunction combines or connects two sentences as one. In the given sentence, the clause "his mother could read it" is a dependent/subordinate clause, as it depends on the independent/main clause for its full meaning. Hence for this sentence, subordinating conjunction is required. A conjunction can be a single word, or a phrase, such as even if, as if, as soon as, etc. We have a pair of conjunctions in this sentence, "so...", where we have to find the right word that pairs up with 'so'. The subordinate clause is the purpose behind which the action in the main clause "wrote carefully" takes place, hence we need conjunction that expresses purpose. That' is the most suitable option, it pairs up with 'so' and expresses purpose, 'that' on its own, it refers to purpose as well, hence option A is correct. 'Than' expresses a comparison between the clauses, it compares two people or things. But here there isn't any comparison, hence option B is incorrect. 'If expresses a condition, or a hypothetical situation, it may or may not be true. However, the given sentence is not conditional, hence option C is incorrect. When' expresses time, it is used when two activities take place at the same time, or one activity begins shortly after the other whereas the given sentence does not focus on time but purpose. Hence option D is incorrect.