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Option 'D' is correct.

Explanation:

We are required to find that length of sides which cannot be sides of triangle,

As we know that " Sum of two sides is always greater than the third side "

If any option does not follow the above postulate that will be our required answer :

D) 21 m, 6 m, 10 m

So, 21 m, 6 m, 10 m cannot be the length of any triangle as it does not satisfy the above mentioned postulate .