The correct option is option 2: Constant proportion. This is because the statement in the question implies that regardless of the source, ammonia always has the same proportion of nitrogen and hydrogen. This observation aligns with the law of constant proportion. According to this law, a given compound always contains the same elements in the same proportions by mass.

Option 1: Reciprocal proportion is not applicable in this context. Reciprocal proportion refers to the relationship between the quantities of two elements in different compounds that combine with a fixed quantity of another element.

Option 3: Multiple proportions is also not applicable in this situation. Multiple proportions refers to the relationship between the masses of one element that combine with a fixed mass of another element in different compounds.

Option 4: None of the above is not the correct answer because option 2, constant proportion, describes the situation accurately.

In conclusion, the fact that ammonia obtained from different sources always has the same proportion of nitrogen and hydrogen supports the law of constant proportion.