Read the passage below and answer the question that follows.
With Agrippa as the best craftsman under the leadership of Augustus, the buildings throughout the Roman Empire had become most magnificent (1). While bringing about change in the imperial landscape, Agrippa became a trusted advisor to Augustus (2). With Agrippa being equipped with a great skill set and mindset, people were able to have observed the ambitious expansion plan of Augustus (3).
Based on the passage, in sentence 3, what is the Latin form of "to have observed"?
a. Observare
b. Observavisse
c. Observatum esse
d. Observaturum iri