William adds two values, following the rules for using significant figures in computations. he should write the sum of these two numbers by using
the same number of decimal places as the least precise value.
the same number of decimal places as the most precise value.
a range of the number of significant figures in the two values.
an average of the number of significant figures in the two values.

the same number of decimal places as the least precise value.

Explanation:

Significant figures : The figures in a number which express the value -the magnitude of a quantity to a specific degree of accuracy is known as significant digits.

The rule apply for the addition and subtraction is :

The least precise number present after the decimal point determines the number of significant figures in the answer.

For example: 12.1 + 3.245 = 15.3  as the least precise number 12.1 has one decimal place.

Thus William should write the sum of these two numbers by using the same number of decimal places as the least precise value.