Final answer:
Augustine felt envious upon witnessing the profound transformation in his friend after his baptism, as detailed in the 'Confessions', triggering Augustine's own introspection about faith.
Step-by-step explanation:
Augustine's initial reaction to his friend's baptism is d) Envious. In his autobiographical work Confessions, Augustine recounts his internal struggles with faith and his journey towards Christianity. When his friend falls ill and requests baptism as a Christian, Augustine is taken aback by the sudden change in his friend, especially upon his recovery, noticing a profound transformation in his beliefs and character. This change evokes a sense of envy in Augustine, as he grapples with his own spiritual insecurities and searches for the same conviction and faith his friend has found through baptism.