This strengthens the argument by reinforcing the assumption that feeding cows better diets would not limit their production of meat or milk. (A) says cows would produce less methane but even more milk and meat.

This does not affect the argument. Cow feed having carbon and hydrogen doesn’t mean anything for our argument, as we know that healthier feed makes them produce less methane.

This does not affect the argument. The conclusion is not about whether farmers should or could give their cows healthier diets. The conclusion is about what would happen if they did.

This does not affect the argument. The conclusion is still valid—regardless of which cows produce more methane, all cows’ methane production would decrease if given healthy diets.

This does not affect the argument, which is solely about methane. The conclusion that methane production would decrease is not impacted by whether methane or carbon dioxide contributes more to global warming.