Yeast is an important source of vitamin B. Vitamin B is a group of water-soluble vitamins that play crucial roles in various processes in our body, such as cell metabolism, the production of red blood cells, and the proper functioning of the central nervous system.

Option 2, invertase, is incorrect as invertase is actually an enzyme produced by yeast and other microorganisms, which helps convert sucrose into glucose and fructose.

Option 3, vitamin C, is not obtained from yeast. Vitamin C is commonly found in citrus fruits and some vegetables.

Option 4, protein, is correct to some extent as yeast does contain protein. However, the primary value of yeast lies in its ability to produce vitamin B rather than being a significant source of protein.

Therefore, the correct answer is option 1, vitamin B.