To increase the acceleration of the dog sled, the best option is to add another dog to the sled.
Adding another dog would increase the total force being exerted on the sled, which can lead to greater acceleration according to Newton's second law of motion (F=ma). The other options, such as removing a dog, running the sled up a hill, or adding more mass to the sled, would either decrease the force exerted or increase the mass without a corresponding increase in force, resulting in decreased acceleration.