Final answer:
The ratio of the mother's length to the calf's length is 3:1.
Step-by-step explanation:
To write the ratio of the mother's length to the calf's length, we divide the mother's length by the calf's length. In this case, the mother's length is 12 feet and the calf's length is 4 feet. So, the ratio is 12/4. Simplifying the ratio gives us 3/1. Therefore, the correct ratio is b. 3:1.