The correct option is C 6
To find the greatest possible length of each plank, we need to find the HCF of 24 and 42.
Factors of 24: 1, 2, 3, 4, 6, 12 and 24.
Factors of 42: 1, 2, 3, 6, 7, 14, 21, and 42
Common factors of 24 and 42: 1, 2, 3, 6
HCF of 24 and 42 equals to 6.
Therefore, the greatest possible length of each plank can be 6 m.