Final answer:
The gestation period in a mare is option D, which is 326 - 354 days, significantly longer than humans (about 266 days) and sheep (about 150 days).
Step-by-step explanation:
The gestation period in a mare, which is the length of time from conception to birth, is option D. 326 - 354 days. This is the period that encompasses the entire development of the fetus inside the mare until it is ready to be born. Comparatively, while the gestation for human pregnancy is approximately 266 days, and for female sheep about 150 days, the mare has a much longer gestation period. Thus, understanding the gestation period of different animals can be important for managing their reproduction and health.