When writing a formula for a compound having a transition metal in it, . it is sometimes necessary to use the Roman numeral as the assigned charge of the metal
it is sometimes necessary to include a Roman numeral after the symbol of the metal
it is always necessary to include a Roman numeral after the symbol of the metal
it is always necessary to use the Roman numeral as the assigned charge of the metal

-please explain

it is always necessary to use the roman numeral as the assigned charge of the metal.

Explanation:

This is so that one would know which Transition metal is being used. For example copper (II) would be Cu²+

the equation that correctly represent the reaction for formation of ammonia is

n2+ 3h2 → 2nh3 (answer d)

1 mole of nitrogen gas (n2) react with 3 moles of hydrogen gas (h2) to form 2 moles of ammonia ( nh3). this is in a process known as haber process were iron is used as a catalyst and reaction take place that a higher temperature and pressure. the process is exothermic hence energy is released.