Its A because it is measuring the puppies weight not just at birth but over a long amount of time so it can’t be B. It’s not just for a specific day so it can’t be C. It’s clearly not just showing the weight of the puppies so it can’t be D. It’s a comparison of both the weight and the amount of days that go by and the line of best fit is used to approximate how heavy a puppy would be on any day if you were to put in your own data. Hope this helps.