Which galaxy is noted for having the most distant observable Cepheid variable stars?

The galaxy that is noted for having the most distant observable Cepheid variable stars is indeed NGC 4603. This galaxy is part of the Hydra constellation and is categorized as a barred spiral galaxy. Cepheid variable stars are crucial for measuring cosmic distances due to their well-defined relationship between their luminosity and their pulsation period. This relationship allows astronomers to determine distances to galaxies more accurately.

NGC 4603's Cepheid variables have been observed at a significantly greater distance compared to those in other galaxies, making them an important target for studies related to the expansion of the universe and the calibration of the cosmic distance ladder. The observation of these distant Cepheids also helps in refining the Hubble constant, which describes the rate of expansion of the universe.

Understanding the significance of Cepheid variables in NGC 4603 emphasizes not just the importance of this specific galaxy in distance measurement, but also underscores the incredible ability of modern telescopes and observational techniques to study distant objects in our universe.

The other options, while they may contain variable stars, do not have the same recognition or record for hosting the most distant observable Cepheid variables, thereby highlighting the unique status of NGC 4603 in this context.