The following sentence has been divided into parts. One of them may contain an error.
Thirty miles / are / a long way to walk.

Correct option is C

Option (c) contains an error.
The error lies in the use of "are" with "thirty miles," which is treated as a singular unit of measurement here, thus requiring the singular verb "is."
Grammatical rule used: When discussing distances, periods of time, sums of money, etc., as units, the singular form of the verb is typically used even if the noun itself is plural.
Example: Fifty dollars is too much to pay for that shirt.