B cells respond to the initial antigen challenge by .

B cells respond to the initial antigen challenge by producing Progeny cells which are  Plasma cells and Memory cells

Explanation:

When a B-cell encounter an antigen, It binds to the B-cell antigen receptor which delivers the antigen to intracellular sites where it is degraded and returned to the B-cell surface as peptides bound to MHC class II molecules.

The peptide: MHC class II complex can be recognized by antigen-specific armed helper T cells, stimulating them to make proteins that, in turn, cause the B cell to proliferate and its progeny to differentiate into antibody-secreting cell. After several rounds of proliferation, B cells can further differentiate into antibody-secreting plasma cells. The germinal B cell can either differentiate as plasma cell which produces antibody or as memory B cell which acts as the memory for further infection.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.