Give a stream of consciousness and then the final answer. The theory states that in a pre-existing group of lymphocytes (specifically B cells), a specific antigen only activates (i.e. selection) its counter-specific cell so that particular cell is induced to multiply (producing its clones) for antibody production. This activation occurs in secondary lymphoid organs such as the spleen and the lymph nodes. In short the theory is an explanation of the mechanism for the generation of diversity of antibody specificity. The first experimental evidence came in 1958, when Gustav Nossal and Joshua Lederberg showed that one B cell always produces only one antibody. The idea turned out to be the foundation of molecular immunology, especially in adaptive immunity.

Answer this question: where does clonal selection of t cells happen?
To answer the above question, the relevant sentence is: This activation occurs in secondary lymphoid organs such as the spleen and the lymph nodes.
So, the final answer is secondary lymphoid organs such as the spleen and the lymph nodes.