(A) $\rightarrow a^n b^n c^m$ is not CFG. NOT correct. $a^n b^n c^m$ is accepted by one stack PDA and generated by CFG.
(B) $\rightarrow a^m b^n c^n$ is deterministic CFG (TRUE)(CORRECT). $a^m b^n c^n$ is accepted by one stack PDA and generated by CFG.
(C) $\rightarrow a^n b^n$ is CFG - TRUE. (CORRECT). $a^n b^n$ is accepted by one stack PDA and generated by CFG.
(D) $\rightarrow a^{800}b^{800} c^{800}$ is CFG - TRUE (CORRECT).