A vehicle is hard to steer in one direction only. Which of the following could be the cause?

When a vehicle is hard to steer in only one direction, it suggests that there is a problem impacting the steering mechanism's ability to assist in turning. A faulty power steering gear can cause this issue because it is responsible for translating the rotational input from the steering wheel into linear motion at the wheels. If the gear is malfunctioning, it may not provide adequate movement or may create excessive friction when turning in one direction, leading to difficulty in steering.

In contrast, while a tight joint in the steering shaft could also potentially lead to steering issues, it typically affects both directions rather than being directional specific. A loose joint might cause play in the steering but would not necessarily make turning in one direction specifically more difficult. A faulty power steering pump could lead to overall loss of steering assistance but usually affects both directions rather than causing a specific directional problem. Therefore, the faulty power steering gear is the most reasonable cause for experiencing hard steering in just one direction.