Which of the following holds the smallest number of decimals when adding significant figures?

When dealing with significant figures, the rules for addition dictate that the result should be reported with the same number of decimal places as the measurement that has the fewest decimal places. Therefore, when you consider the phrase "smallest number of decimals," it directly corresponds to this rule in significant figure arithmetic.

For instance, if you are adding measurements such as 12.1 (one decimal), 0.234 (three decimals), and 5.5 (one decimal), you would round the final result to one decimal place because 12.1 and 5.5 both have only one decimal digit.

This rule emphasizes the importance of maintaining the precision of the least precise measurement in the operation, thereby ensuring that the final answer isn’t artificially precise beyond the limitations imposed by the original data. Hence, the choice that emphasizes the smallest number of decimals aligns perfectly with the principles of significant figures in addition.