Compare the numbers to find the greatest.<br><br>&bull; List the numbers: 21, 20, 22, 23<br>&bull; Compare tens: all have 2 tens (twenties).<br>&bull; Compare ones: 1, 0, 2, 3 &mdash; the largest ones digit is 3.<br>&bull; So 23 is the greatest.<br><br>Final Answer: D: 23