Final answer:
A "double-negative" thymocyte is present before both positive selection and negative selection, corresponding to the option 'b. before ; before' for the stages of T cell development in thymus.
Step-by-step explanation:
The student has asked about the presence of "double-negative" thymocytes during the stages of T cell development in the thymus. These double-negative thymocytes are present during the initial stage of T cell development, which means they are present before positive selection. This is the phase when thymocytes are in the cortex of the thymus and do not yet express the CD4 or CD8 molecules. After undergoing positive selection and becoming double-positive (expressing both CD4 and CD8 markers), thymocytes move to the junction between the cortex and medulla for negative selection. Consequently, a "double-negative" thymocyte would be present before positive selection but not after negative selection: hence the answer is option 'b' before; before.