Since 1954, when the Supreme Court ruled that segregation

Since 1954, when the Supreme Court ruled that segregation was unconstitutional, the Supreme Court heard cases in which lawyers debated what constituted segregation.

The question tests the knowledge on

Meaning clarity, Tenses, Verb Usage

Meaning
Supreme Court ruled segregation was unconstitutional (At The Past)
From that time, there has been intense debate in Supreme Court as what can be categorized as constitution segregation (At past and present)

POE
Please consider the Red flag "Since"- Present perfect should be used
Options A, B, E can be eliminated due to the wrong usage of tenses

(A) heard cases in which lawyers debated what constituted segregation [Past]
(B) had heard cases in which lawyers debated what constituted segregation [Past perfect]
(C) has heard cases in which lawyers debated what constituted segregation
(D) has heard cases in which lawyers debated what has constituted segregation
(E) was hearing cases in which lawyers debated what constituted segregation [Past Continuous]

Between C and D
D. There is an ambiguity in meaning. This option says that the lawyers heard only the cases in which there has been a debate on constituted segregation
Hence Option C