Final answer:
The Sierra Madre Oriental, an extension of the Rocky Mountains, running approximately 808 miles along Mexico's east coast, is the mountain range with a length closest to 750 miles.
Step-by-step explanation:
The mountain range that is only 750 miles long and relevant to the provided information would be the Sierra Madre Oriental, which is an extension of the Rocky Mountains and runs approximately 808 miles along the east coast of Mexico. Neither the Sierra Madre Occidental, which is about 3,107 miles long nor the Sierra Nevada range, which runs 400 miles across California, fit the provided length. Therefore, the Sierra Madre Oriental, being closest to 750 miles in length among the mentioned ranges, is the correct answer. It is worth noting that no mountain range is exactly 750 miles long according to the information provided, but the Sierra Madre Oriental's length is the closest to that figure.