Final answer:
In the sentence 'This old palace is supposed to have been built 50 years ago,' the proper phrase to use is 'to have been built'. This communicates that the palace's construction was completed in the past and is still relevant in the present.
Step-by-step explanation:
The student's question pertains to the correct use of an infinitive phrase in a sentence describing the construction of a building. The correct answer is 'B. to have been built'. Therefore, the sentence should read 'This old palace is supposed to have been built 50 years ago.' This correct usage of the infinitive form indicates that the building of the palace was completed in the past and it expresses a perfect aspect, which is appropriate here since the construction occurred in the past but has relevance to the present—the palace still exists.
The Palacio del Partal (Partal Palace) serves as an interesting historical example, as it was constructed in the 14th century and is part of the Alhambra palace complex. Similarly, the Castillo de San Marcos was built not for aesthetic or domestic reasons but rather 'B. to defend against imperial challengers', offering a practical example of how such structures were often built with strategic purposes in mind.