Answer A is correct. The relative clause is formed correctly with the use of the pronoun "which" preceded by a comma. The clause "which is over 50 meters" concerns the distance. "The distance" is a thing and pronouns "who" and "whom" are used to refer to people.
Answer B is incorrect because the incorrect pronoun "who" is used.
Answer C - incorrect. Again the incorrect pronoun "whom" appears in the relative clause.
Answer D - incorrect. The pronoun "that" is used in defining relative clauses and not preceded by a comma.
Answer E -the use of "whoever" is incorrect. It can also refer only to people.

little confused here ,

Two rules of "Which" to correct in GMAT is
1. It should always come after comma.
2. It should refer to noun that comes immediately before that comma.

Also which is only explanatory in GMAT .

So considering to above points ,given sentence does not seem correct to me . To me "That" is more appropriate.