The right option is; orbital period
T stands for orbital period.
Orbital period is the time a certain astronomical object such as planets takes to complete one orbit (360° revolution) around another object such as sun or stars. Orbital period is expressed in different units of time, usually in years, days or hours.
I believe this question is trying to get you to realize the difference between acquired traits(one you get through doing something during life) and heritable traits(ones from Mom/Dad). For example, I really wish I was 6 foot 4... but the height trait comes genetically from your parents since it is coded in my DNA how tall I will be, therefore this trait must be passed down genetically and somewhere down the evolutionary tree the humans in the region which my family comes from was better off being short. An example that comes from acquired traits could be muscularity... my parents are both not very muscular but throughout my lifetime I enjoy working out and have become much more muscular, no matter how much your parents or grandparents lift weights YOU won't change so that change must be acquired through aging during your lifetime. Now the amount of muscle your body can put on is a heritable thing but that is beside the point. Hope this helps :)
