**Answer:**

To determine which gas will produce 4 L of H₂O(g) on combustion, we need to calculate the stoichiometric ratios of the gases involved in the reaction.

The balanced equation for the combustion of a hydrocarbon can be written as:

*Hydrocarbon + O₂(g) → CO₂(g) + H₂O(g)*

Since we have excess O₂, we can assume that the hydrocarbon is the limiting reagent in this case.

To find the volume of H₂O(g) produced, we need to consider the ideal gas equation:

*PV = nRT*

Where:
- P is the pressure
- V is the volume
- n is the number of moles
- R is the ideal gas constant
- T is the temperature

We can rearrange the equation to solve for the number of moles:

*n = PV/RT*

Since we're given the pressure, volume, and temperature, we can calculate the number of moles of the hydrocarbon and the number of moles of H₂O(g) produced.

Let's consider each gas option:

1. **Methane (CH₄):**
- The balanced equation for the combustion of methane is: *CH₄ + 2O₂ → CO₂ + 2H₂O*
- From the balanced equation, we can see that for every 1 mole of methane combusted, 2 moles of H₂O are produced.
- Therefore, the number of moles of H₂O produced from 1 L of methane is: *n(H₂O) = 2 x n(CH₄)*

2. **Ethane (C₂H₆):**
- The balanced equation for the combustion of ethane is: *C₂H₆ + 7/2O₂ → 2CO₂ + 3H₂O*
- From the balanced equation, we can see that for every 1 mole of ethane combusted, 3 moles of H₂O are produced.
- Therefore, the number of moles of H₂O produced from 1 L of ethane is: *n(H₂O) = 3 x n(C₂H₆)*

3. **Propane (C₃H₈):**
- The balanced equation for the combustion of propane is: *C₃H₈ + 5O₂ → 3CO₂ + 4H₂O*
- From the balanced equation, we can see that for every 1 mole of propane combusted, 4 moles of H₂O are produced.
- Therefore, the number of moles of H₂O produced from 1 L of propane is: *n(H₂O) = 4 x n(C₃H₈)*

To determine which gas will produce 4 L of H₂O(g) on combustion, we need to compare the moles of H₂O produced from 1 L of each gas. The gas that produces the highest number of moles of H₂O will be the answer.

Let's calculate the number of moles of H₂O produced from 1 L of each gas at 373 K and 1 atm pressure:

1. Methane (CH₄):
- Using the ideal gas equation: *n(CH₄) = PV/RT = (1 atm x