The method to calculate Tension differs in every case. This is why answer B is wrong. There is also no specific formula to find Tension, which is why D is incorrect. Instead, we must come up with our own unique equations based on Newton’s 2nd law, which states F net = ma (this is why choosing A is correct). In other words, we simply pick a direction that Tension acts in and add up all the forces (including Tension) and set everything equal to ma. The reason B is incorrect is because Tension is only affected by the forces that are acting in the same direction as Tension.