Bull semen is stored in

The correct option is C Liquid nitrogen
Ideal semen storage conditions include low pH, hyperosmotic pressure, warm temperature and low oxygen tension with high sperm density. This makes option A incorrect as anaerobic environment is lethal for sperms. Option B is also incorrect as high oxygen tension is not suitable for sperms. Freeze drying technique of sperm storage stores bull sperms in dry ice at 4 degree Celsius which maintains its viability and conception rate up to 3-4 years. However, storage of bull sperm in liquid nitrogen gives the conception rate as high as that of fresh semen and also is a simpler, less expensive technique than freeze drying which makes option D incorrect.

So, the correct answer is option C.