The correct answer is: A. Twice the diameter.
The radius of a bar bend to form a hook should not be less than twice the diameter of the bar. This is because the radius of the bend is the distance from the center of the bar to the outside edge of the bend. If the radius is too small, the bar will be under too much stress and could break.
Option B, thrice the diameter, is too large. A radius of three times the diameter would make the hook very strong, but it would also be very large and difficult to use.
Option C, four times the diameter, is also too large. A radius of four times the diameter would make the hook even stronger than a radius of three times the diameter, but it would also be even larger and more difficult to use.
Option D, five times the diameter, is much too large. A radius of five times the diameter would make the hook incredibly strong, but it would also be incredibly large and impractical.
In conclusion, the radius of a bar bend to form a hook should not be less than twice the diameter of the bar.