Final answer:
The correct formula for ammonium nitrate is NH4NO3, represented by option (a). It is composed of the ammonium ion and the nitrate ion, and has applications in both agriculture and explosives.
Step-by-step explanation:
The formula for ammonium nitrate is NH4NO3, which is option (a) NH4NO3. This compound is an ionic compound consisting of the ammonium ion (NH4+) and the nitrate ion (NO3−). The compound is widely used in agriculture as a fertilizer due to its high nitrogen content. It's also known for its use in explosives under certain conditions due to its capability to rapidly decompose into gases, as described by the decomposition reaction NH4NO3 (s) → N2O(g) + 2H2O(g).