Plessy v. Ferguson, decided in 1896, ruled that "separate but equal" was legal. This decision upheld state laws requiring racial segregation in public facilities, as long as the separate facilities were considered equal. Therefore, the correct answer to your question is:
"Separate but equal" was legal.
The other options listed are incorrect in the context of the ruling of Plessy v. Ferguson. The Grandfather Clause and claims about segregation violating the 14th or 15th Amendments were related to different legal challenges and decisions.