We marveled-------- the beauty of the monument. ?

The correct choice for the sentence "We marveled-------- the beauty of the monument." is D) at. The verb "marveled" is commonly used in conjunction with the preposition "at" when expressing admiration or wonder towards something. This specific prepositional phrase (marveled at) indicates awe or astonishment directed towards a particular subjectin this case, "the beauty of the monument."

Using "at" in this context correctly conveys that the admiration is focused on the beauty itself. For example, we say, "He marveled at the breathtaking landscape," rather than "marveled to" or "marveled up." The other options do not fit grammatically or semantically. "Up" and "to" do not appropriately associate with feelings of wonder, while "of" would incorrectly suggest that the verb directly describes a relationship with the beauty, rather than the action of being amazed by it.

In summary, "marveled at" is the standard expression for expressing wonder or admiration, making "at" the correct preposition to complete this sentence.