Explanation:
To determine the species with a zero standard molar enthalpy of formation at 298 K, we need to understand the concept of standard molar enthalpy of formation.

Standard molar enthalpy of formation:
The standard molar enthalpy of formation (∆H°f) of a compound is the enthalpy change when one mole of the compound is formed from its constituent elements in their standard states, with all reactants and products at 298 K and 1 atm pressure.

Key Point:
The standard state of an element is the most stable form of the element at 298 K and 1 atm pressure.

Now, let's consider the options given and analyze each one:

a) Br2(g):
Bromine gas is in its standard state at 298 K and 1 atm pressure. Therefore, the standard molar enthalpy of formation of Br2(g) is not zero.

b) Cl2(g):
Chlorine gas is in its standard state at 298 K and 1 atm pressure. The formation of Cl2(g) from its constituent elements (Cl) does not involve any change in enthalpy, as they already exist as diatomic molecules. Hence, the standard molar enthalpy of formation of Cl2(g) is zero.

c) H2O(g):
Water vapor is not in its standard state at 298 K and 1 atm pressure. The standard state of water is liquid at 298 K and 1 atm pressure. Therefore, the standard molar enthalpy of formation of H2O(g) is not zero.

d) CH4(g):
Methane gas is not in its standard state at 298 K and 1 atm pressure. The standard state of methane is gas at 298 K and 1 atm pressure. Therefore, the standard molar enthalpy of formation of CH4(g) is not zero.

Conclusion:
Among the given options, the species with a zero standard molar enthalpy of formation at 298 K is option 'b) Cl2(g)'.