Final answer:
The true statement is that the direction of the magnetic field is always tangent to the field lines, with a compass pointing along these lines at any given point.
Step-by-step explanation:
The statement that must be true concerning the constructions shown is that the direction of the magnetic field is tangent to the field line at any point in space, and a small compass will point in the direction of the field line. This is demonstrated by the behavior of small compass needles when brought near different magnetic field configurations, such as those created by a circular current loop, a long and straight wire, or a wire in the plane of the paper, as indicated in Figure 22.15. Additionally, when considering the Earth's magnetic field, the north pole of a compass needle is attracted to the south magnetic pole of Earth, which correlates with the geographic North Pole.
The construction Samantha made using a compass and a straightedge can be used to map magnetic field lines. When a small compass is placed in the magnetic field, its needle aligns itself with the field and points along the field line. Therefore, the statement that must be true is: The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.