What do two solid yellow lines indicate?

Answer: D — No passing is allowed

Two solid yellow lines indicate no passing is allowed. This is because on roads with two solid yellow lines, there is limited visibility and it is unsafe to attempt to overtake or pass other vehicles. Passing in this situation increases the risk of accidents and potential collisions, so it is important to closely follow and abide by these traffic markings. Option A is incorrect because passing is NOT allowed. Option B is incorrect because it does not specify whether passing is allowed or not. Option C is incorrect because it describes traffic moving in opposite directions, but does not mention anything about passing.