Who was the great Roman architect of the time of Augustus, known for writing "de Architectura"?

The great Roman architect of the time of Augustus who wrote "De Architectura" is indeed Vitruvius. This work is significant as it is one of the earliest and most comprehensive texts on architecture and engineering in the Western world. In "De Architectura," Vitruvius discusses various topics such as building materials, the principles of design, urban planning, and the importance of proportion and symmetry in architecture.

Vitruvius emphasized the importance of functionality, durability, and beauty in buildings, principles that have influenced architectural theory and practice for centuries. His writings not only provide insight into the architectural practices of his time but also reflect the cultural and technological advancements of Rome during Augustus's reign. This makes Vitruvius a foundational figure in both architecture and the historical study of Roman engineering.

The other individuals listed—Pliny the Elder, Haraldur Sigurdsson, and Seneca—had different focuses. Pliny the Elder was known for his encyclopedic work on natural history, Seneca was a philosopher and playwright, and Haraldur Sigurdsson is not a recognized figure in Roman architecture or history. This contextual understanding highlights Vitruvius's unique contributions to architecture, establishing him as the correct answer to the question.