Who is the god associated with male procreative power and the guardian of gardens and vineyards in Roman mythology?

The correct answer is Priapus, who is specifically associated with male procreative power and is often depicted as a fertility god in Roman mythology. He is known for being the protector of gardens, vineyards, and livestock, symbolizing abundance and fertility. Typically represented with exaggerated features, Priapus embodies the principles of fertility and the growth of crops, making him vital in agricultural contexts.

While Jupiter is a major deity associated with sky and thunder, and Neptune governs the seas, neither of these gods focuses on the specific themes of male fertility and garden protection embodied by Priapus. Mars, as the god of war, also does not align with the themes of fertility and agricultural guardianship, which are central to Priapus's character. Hence, Priapus stands out as the deity linked directly to the aspects of procreative power and the stewardship of nature, making him the correct answer.