S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the C(5) position of a cytosine in 25S rRNA. Required for 60S ribosomal subunit synthesis and processing (By similarity). a cytidine in 25S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 25S rRNA + H(+) + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.