The answer is the first one. That's because the general theory of relativity is the thing experiencing whatever is experiencing relative to something else. The second answer is just plain wrong. The third answer is just a constant, and doesn't relate to experiencing anything. And the fourth answer is a force between two objects, and it has no second comparison. The first answer is how a subject experiences two different things.
Being in an accelerated frame is indistinguishable from experiencing gravity.
Explanation:
The general theory of relativity is the geometric theory of gravitation that describes it in modern physics. It generalizes the gravity as a geometric property of the space and time, or spacetime.
One of the principles of this theory is the equivalence principle, which states that the gravitational force experienced at a stade mode have no difference of the pseudo-gravity experienced in an accelerated frame, if the referential is a massive body, such as Earth.
Mass is not equivalent to weight, because weight is a force, which is the mass multiplied by the gravity acceleration; the other two statements come from Newton's gravitational theory.
