Final answer:
The area of San Bernardino County is 19,683 square miles, expressed as 19,683 miles², which is answer choice d).
Step-by-step explanation:
The area of San Bernardino County in California is 19,683 square miles. When expressing this numerically, you can simply write it as 19,683 with the word 'miles' indicating the unit of area, and you do not need any additional symbols like multiplication or division. Therefore, the correct answer is d) 19,683 miles². It's essential to note that the symbol for square miles is miles², which represents the area of a two-dimensional space measured in miles.