Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of nonfinite verb forms in a sentence. The nonfinite to-infinitive verb "to join" is correctly used to form a subordinate clause that expresses why Ochoa left her position (to join the training program).
Why the other choices are wrong
Choice B
Choice B is incorrect because the finite verb "is joining" can’t be used in this way to indicate Ochoa’s action of joining the training program. A conjunction such as "and" would be needed to coordinate "is joining" with the previous finite verb, "left."
Choice C
Choice C is incorrect because the finite verb "joined" can’t be used in this way to indicate Ochoa’s action of joining the training program. A conjunction such as "and" would be needed to coordinate "joined" with the previous finite verb, "left."
Choice D
Choice D is incorrect because the finite verb "joins" can’t be used in this way to indicate Ochoa’s action of joining the training program. A conjunction such as "and" would be needed to coordinate "joins" with the previous finite verb, "left."