The correct answer is D. no article.
The River Thames is a river in England and the longest river entirely in England and Wales. It rises in Gloucestershire, flows through Oxfordshire, Berkshire, Buckinghamshire, Surrey, and Greater London, and empties into the North Sea at Southend-on-Sea.
The River Thames is a major transport artery, and London is a major port city. The river is also a popular tourist destination, and there are many boat tours available.
The article "a" is used before singular nouns that refer to a general class of things. For example, "I saw a bird." The article "an" is used before singular nouns that begin with a vowel sound. For example, "I saw an apple." The article "the" is used before singular nouns that refer to a specific thing. For example, "I saw the bird that flew away."
In the sentence "London is on the River Thames," the word "Thames" is a proper noun, which is a noun that refers to a specific person, place, or thing. Proper nouns are always capitalized. In this case, the word "Thames" refers to a specific river, so the article "the" is not used.
