Never make a U-turn:

1. in residential districts.
2. at or on a railroad crossing. ✓
3. where there isn't a "No U-turn" sign posted.

Answer: B. at or on a railroad crossing.

Never make a U-turn at or on a railroad crossing.

Why this is the answer

The handbook lists places where U-turns are never allowed, and one listed place is at or on a railroad crossing. It also says you may make a U-turn in a residential district when no vehicles are approaching within 200 feet, and at an intersection on a green light or arrow unless a NO U-TURN sign is posted. So the blanket rule applies to the railroad crossing, not to residential areas or to places where no sign is posted.

Why the other answers are wrong

• A. in residential districts. — A U-turn is not banned outright in residential districts; the handbook allows it there when no vehicles are approaching within 200 feet.
• C. where there isn't a "No U-turn" sign posted. — The absence of a NO U-TURN sign does not create a ban; the handbook bans U-turns where such a sign is posted and allows them at some intersections without one.

Remember it

Picture the tracks as a line you never loop back across: if you would turn around on or at the crossing, keep going straight.