Which compound is formed from its elements by an exothermic reaction at 298 k and 101.3 kpa?

STP means standard temperature and pressure at 0°C (273K) and 1 atm (atmosphere). The density of the unknown gas is 0.63 gram per liter. The deal gas equation is PV = nRT. The n is the numer of moles and can be represented as mass of the gas, m, divided by the molar mass, c.  so we have,

PV = nRT

PV = (m/c)RT

Since the density is d = m/V

Pc = (m/V)RT

Pc = dRT

c = drT/P

substitute the values into the equation,

c = [(0.63g/L)(0.08206 L-atm/mol-K)(273K)]/(1atm)

c = 14.11 g/mol

it is b that is the answer.