What is the diameter, in miles, of the largest observed comet?

The largest observed comet, known as Comet Hale-Bopp, has been measured to have a nucleus with a diameter of approximately 25 miles. This measurement takes into account the size of its solid core, which is made up of ice, dust, and other materials. Hale-Bopp was particularly noteworthy due to its brightness and visibility from Earth, which contributed to the interest in its dimensions.

When considering the other options, while the observed sizes of certain comets can vary, the consensus in astronomical observations places Hale-Bopp's diameter closest to 25 miles, making this the most accurate choice. The other options significantly overestimate the size, as no other observed comet has been documented with a nucleus exceeding 30 miles in diameter. Therefore, the selection of 25 miles is supported by established astronomical data.