The concept of conjunctions is required to determine the correct answer in the given sentence. So-that is a subordinating conjunction that connects the main and the independent clause to express the cause or reason.
Option a,but, is used to suggest a contrast. Thus option a is incorrect.
Option c,then, indicates time. Thus option c is incorrect.
Option d,so, is incorrect as so-so is not a correct subordinating conjunction and cannot be used to link the clauses.
Thus option b,that, is the correct answer.