The most impressive monument of Egypt's greatness, and one of the wonders of the world, are the pyramids.

This should be '...is the pyramids'.

This is because the subject of the sentence is 'The most impressive monument'.

'Monument' is a singular concept, even though there is more than one pyramid.

That is – 'The most impressive monument is the pyramids'.

[NB! This comes from Samuel Johnson, by the way. Even the great sometimes make mistakes.]

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