The correct option is C 343√3
We know that Surd is nth root of a positive number which can not be simplified into a rational number.
5√100000=(105)15=10
Hence, it is not a surd.
4√81=(34)14=3
Hence, it is not a surd.
343√3=(3)1343
Hence, it is a surd.
3√343=(73)13=7
Hence, it is not a surd.