What does a single, solid yellow line painted on the roadside indicate?

Answer: C — Passing is prohibited

If a single, solid yellow line is painted on the roadside, it indicates that passing is prohibited. In most countries, yellow lines generally indicate a restriction, such as no passing, no stopping, or no parking. Therefore, options A and D can be ruled out. Option B is also incorrect because a solid yellow line does not necessarily indicate that you should keep to the right, as it can also be used to indicate a center lane or a turn lane. Therefore, the best answer is option C, passing is prohibited.