What does annex zeros to align place values mean?

The correct answer is:

When you "annex" something, you add it to something else.

If you annex zeros, you are sticking zeros onto a number.

Generally this is done when adding or subtracting decimals. When performing these two operations on decimals, we must have the decimal point lined up. This is sometimes easier to do if all of the numbers have the same number of digits; if there are digits missing after the decimal, we can add zeros to the end and not change the value of the number. Then the number of digits will be the same and the decimals will be aligned.

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the answer is d