Final answer:
Gravitational field lines point towards masses, electric field lines point from positive to negative charges, and magnetic field lines form loops around magnets.
Step-by-step explanation:
In the case of gravitational field lines, they point towards masses, indicating the direction in which an object would be pulled due to gravity. For electric field lines between a positive and negative charge, the lines originate from the positive charge and terminate on the negative charge.
In the case of electric field lines between two positive charges, the lines repel from each other, creating a pattern of diverging lines. In the case of magnetic field lines around a magnet, they form loops that go from the north pole to the south pole of the magnet.
The entities responsible for the gravitational field lines are masses, for electric field lines they are positive and negative charges, and for magnetic field lines they are magnets.