Answer:
The Tower of London
Step-by-step explanation:
The underlined phrase in the given sentence is is older than Westminster Abbey The noun phrase that is modified (described) by it is the Tower of London.
A noun phrase is a group of words that contains a noun and functions like one. This means that it can function as the subject, object of a verb, object of a preposition, or a complement within a sentence. The main noun in the given phrase is Tower, and this is why we could say that Tower is the noun modified by the underlined phrase.