The error is in part (a), which mentions "50 meters are the". The correct form should be "50 meters is the".
The rule in English is that when a plural noun ('50 meters') denotes a specific amount, length, or quantity considered as a whole, the verb following it is generally singular. So '50 meters' should be followed by the singular verb "is" instead of the plural verb "are".
Hence, (a) is the correct answer.
The corrected sentence: 50 meters is the distance from the starting line to the first hurdle in a track race.