Final answer:
Increasing the temperature option a, of the equilibrium reaction CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g) will shift the reaction to the left, favoring the reactants.
Step-by-step explanation:
The chemical equation CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g) + heat represents a reaction at equilibrium in a closed flask. The action that will cause the reaction to shift to the left, according to Le Chatelier's principle, is increasing the temperature. This is because the reaction is exothermic (releases heat), and increasing the temperature will favor the endothermic reaction (absorbing heat), which is the reverse process. Option a.