Short Answer 
Gravitational potential energy is usually defined as zero at an infinite distance from the gravitational source, which makes option C the correct choice. This convention offers consistency and mathematical convenience in physics, although in specific contexts other reference points may also be used.
Step 1: Understanding Gravitational Potential Energy
Gravitational potential energy is the energy stored in an object due to its position in a gravitational field. The reference point for measuring this energy can be chosen arbitrarily; however, the conventional choice is to set the value at an infinite distance from the gravitational source to zero. This is widely accepted in the field of physics for consistency and mathematical convenience.
Step 2: Evaluating Reference Points
When assessing different points of reference for gravitational potential energy, it’s essential to understand that not every point is universally applicable. Consider the following options:
- A) x = 0: This is not universally correct, as it depends on the situation.
- B) The potential energy can be nonzero even when the gravitational force is constant.
- C) An infinite distance from the gravitational source is the universally accepted point for defining potential energy as zero.
Step 3: Conclusion on Choosing the Zero Point
In summary, while specific situations might allow for different zero points (like the Earth’s surface), the standard choice to define gravitational potential energy as zero is at an infinite distance from the gravitational source. This ensures that as separation increases, gravitational effects diminish, reinforcing why option C is the correct answer.