The correct answer is A. When the subject of a sentence is singular, add "_s" to the base form.
An easy example is "to exist", the base form is "exist" and A strawberry exists shows that you add s when a singular subject is used.
C is the correct answer I believe
Answer: A. George Stephenson was joined by other inventors to enhance the design of locomotives.
Explanation: I got it right on Edmentum.