Final answer:
Smith envies the lunatic due to his freedom from worldly and social pressures, which allows for a liberation that Smith longs to experience.
Step-by-step explanation:
Smith 'envies' the lunatic who wanders on the cliff primarily a) Because he is free from worldly concerns. The lunatic, in his madness, is pictured as being liberated from the materialistic and social pressures that bind those who are considered sane. This freedom from the typical burdens and constraints of society fascinates Smith, and he longs for a similar form of escape. Although option d) Because he is challenging socially accepted norms might also be a factor, it's the broad liberation from societal expectations and materialistic chains that Smith finds compelling.
Several excerpts from literature echo this sentiment, portraying characters who yearn for or experience a disconnect from society and its constructs, whether due to madness, a desire to break away from social norms, or disillusionment with the modern world. They all share a common thread of seeking freedom from societal constraints and finding a sort of terrible purity in separation or madness.