(A) He claims he scored 800 on his GMAT.
(B) He claims that he scored 800 on his GMAT.
B is correct. Do not eliminate 'that' in GMAT
including, regarding, concerning, according to, during
Everyone brought me a present, but she didn’t (before clause) - conjuntion
I was late for I had been stuck in traffic (before clause) - conjunction
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