The standard molar enthalpy of formation, denoted as $$\Delta H_f^\circ$$, is defined as the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (298 K and 1 atm pressure). By definition, the standard molar enthalpy of formation of a pure element in its most stable form at 298 K is zero.

Looking at the options:

• Br₂ (g) - Bromine's standard state at room temperature is liquid (Br₂(l)), not gas.
• Cl₂ (g) - Chlorine's standard state at room temperature is a gas, which means the standard molar enthalpy of formation for Cl₂(g) is indeed zero.
• H₂O (g) - Water in gaseous form is not a basic element but a compound of hydrogen and oxygen, so its enthalpy of formation is not zero.
• CH₄ (g) - Methane, represented here, is also a compound composed of carbon and hydrogen, thus its enthalpy of formation is also not zero.

Therefore, the correct option is Option B, Cl₂ (g), as chlorine gas is a diatomic molecule and an element in its standard state at 298 K, and as such, its standard molar enthalpy of formation is zero.