The correct answer would be the first one - bicycle
A bicycle would be a modern convenience, something that can be acquired by most people, that promotes physical activity as it gets your body moving. An elevator promotes no type of physical activity whereas a staircase used instead would. A chainsaw isn't an item that everybody would own and you wouldn't use a chainsaw long enough for it to make a difference physically. An electric drill does not promote physical activity as you again you would not be using one for long as it doesn't take much effort to use one.