Calculate the greatest possible length which can be used to measure exactly the lengths 220 cm, 260 cm, and 300 cm.

The lengths 220 cm, 260 cm, and 300 cm.

The prime factorization of each number:

​$$220 = 2^2 \times 5 \times 11$$

​​$$260 = 2^2 \times 5 \times 13$$

​$$H.C.F. = 2^2 \times 5 = 4 \times 5 = 20$$​​

So, the greatest possible length is 20 cm.

Thus, the correct answer is (a).