Final answer:
When rounding to 4 significant figures, the number 123 456 789.0 would be rounded to D. 123.4567.
Step-by-step explanation:
When rounding to a certain number of significant figures, we look at the digit that follows the last significant digit. If this digit is 5 or greater, we round up the last significant digit. If it is less than 5, we do not change the last significant digit. If the digit is exactly 5, we round the last significant digit to the nearest even number.
Based on these rules, the number 123 456 789.0 rounded to 4 significant figures would be D. 123.4567. The last digit '9' is followed by a '0', which is less than 5, so we do not change the last significant digit '7'.