Option A is correct because both the assertion and reason are true, and the reason correctly explains the assertion. The context explicitly states that 'Recombinant immunotoxins are rationally engineered protein agents' which confirms the assertion. The reason is also true as the context explains that recombinant immunotoxins are produced by creating single-chain variable fragments (scFv) from antibody variable regions (VH and VL chains) through rDNA technology, and then fusing these with toxin fragments using linkers. This fusion process of antibody-derived DNA with toxin DNA is precisely what makes them 'rationally engineered' protein agents, making the reason the correct explanation for the assertion.