Choose the option that best completes the gap(s) _________ is expanding rapidly.

The option that best completes the gap is "The city of Abuja."

The correct phrase here is "The city of Abuja" because:

• "The city of Abuja" uses the article "the," which specifies a particular city, making the statement clear and precise.
• The word "city" is appropriately placed to refer to Abuja as an urban area, which is necessary to make the sentence grammatically correct and contextually accurate.

Let's compare this with the other options:

• "A city of Abuja" - The article "a" suggests any city, which gives an ambiguous meaning since it doesn't specify that we are talking about Abuja being recognized as a city.
• "City of Abuja" - This lacks an article ("the" or "a"), making the phrase incomplete. Articles are important to make a noun phrase grammatically correct.
• "Abuja of a city" - This arrangement doesn't make grammatical or logical sense. The phrase appears jumbled and confusing.

Therefore, the most grammatically sound and contextually appropriate choice is "The city of Abuja."