Regulatory t cells
a. may function in preventing autoimmune reactions
b. decrease their activity as antigenic stimulus decreases
c. trigger antibody production and stimulate macrophages
d. release cytokines that increase the activity of cytotoxic t cells and activated b cells
e. secrete interferons

The correct answer is option A, that is, may function in preventing autoimmune reactions.  

Explanation:

The Tregs or the regulatory T cells refers to the T cells that exhibit a function in suppressing or regulating other cells in the immune system. They monitor the immune response to foreign and self particles and assist in preventing autoimmune disease. The Tregs generated by a normal thymus is known as natural. They are produced due to the proliferation of naive T cells external to the thymus.