Invaluable is closest in meaning to:

Explanation

The correct answer is option C:Highly expensive. The closest meaning to "invaluable" is "highly valuable" or "priceless". "Highly expensive" may refer to something that has a high cost, but it is not equivalent to "invaluable". "Invaluable" implies that something is so valuable that it cannot be measured in terms of money or price. Options A, B, and D are incorrect because they do not accurately represent the meaning of "invaluable" and do not serve as synonyms for the term.