Explanation:

The standard molar enthalpy of formation, ΔH°f, is the enthalpy change when one mole of a compound is formed from its elements in their standard states at a specified temperature and pressure. The standard state of a substance is the form it is most stable at under standard conditions (298 K and 1 atm pressure).

To determine which species has a zero standard molar enthalpy of formation at 298 K, we need to consider their formation reactions and enthalpy changes.

Formation Reactions:
1. Br2(g): Formation reaction: Br2(g) → Br2(g)
2. Cl2(g): Formation reaction: Cl2(g) → Cl2(g)
3. H2O(g): Formation reaction: H2(g) + 1/2O2(g) → H2O(g)
4. CH4(g): Formation reaction: C(s) + 2H2(g) → CH4(g)

Enthalpy Changes:
1. Br2(g): The formation reaction for Br2(g) does not involve any enthalpy change as it is already in its standard state. Therefore, the standard molar enthalpy of formation of Br2(g) is zero at 298 K.
2. Cl2(g): Similarly, the formation reaction for Cl2(g) does not involve any enthalpy change as it is already in its standard state. Therefore, the standard molar enthalpy of formation of Cl2(g) is zero at 298 K.
3. H2O(g): The formation reaction for H2O(g) involves the combination of hydrogen gas and oxygen gas to form water vapor. This reaction releases energy, resulting in a negative enthalpy change. Therefore, the standard molar enthalpy of formation of H2O(g) is not zero at 298 K.
4. CH4(g): The formation reaction for CH4(g) involves the combination of carbon and hydrogen gas to form methane. This reaction also releases energy, resulting in a negative enthalpy change. Therefore, the standard molar enthalpy of formation of CH4(g) is not zero at 298 K.

Therefore, the species that has a zero standard molar enthalpy of formation at 298 K is Cl2(g) (option B).