The correct answer is option 2: conservation of mass-energy. This law states that mass and energy cannot be created or destroyed, only transformed from one form to another. In other words, the total amount of mass and energy in a closed system remains constant.

Option 1: constant proportions is not applicable to this scenario as it refers to the law of definite proportions, which states that a compound will always have the same ratio of different elements.

Option 3: multiple proportions is also not related to the question as it refers to the law of multiple proportions, which states that when two elements form different compounds, the ratio of their masses will be a small whole number.

Option 4: conservation of momentum is not relevant to the creation of something from nothing, as momentum refers to the quantity of motion and is not related to the creation of matter or energy.

Therefore, the most appropriate answer to the question is option 2.