The correct option is D NO CHANGE
A subordinate clause ("we may get admitted to good universities") depends on the main clause ("We study hard") for its complete meaning. To combine these two as a sentence, a subordinating conjunction is used. The subordinate clause here presents the purpose or reason for which something is done, the purpose is 'to get admitted in a good university' and what needs to be done to achieve this is 'to study hard'. "So that" is a conjunction used for purpose-related sentences, as the given one, hence no change is necessary, and option D is correct. 'Yet' is a coordinating conjunction that expresses contrast between two sentences, it combines two independent sentences, that don't rely on each other for their meaning to be complete. The given sentence has a dependent clause, and does not have a contrast between the clauses, hence 'yet' cannot be used and option A is incorrect. 'But' and 'and' are also coordinating conjunctions, and the given sentence requires a subordinating conjunction, hence options B and C are incorrect.