Step 1: A bacterial ribosome (70S) splits into two unequal subunits based on sedimentation behavior in a centrifuge: a smaller 30S piece and a larger 50S piece.
Step 2: The 30S piece has one rRNA, 16S. The 50S piece is bigger and holds two separate rRNA strands, a large 23S rRNA and a small 5S rRNA.
Step 3: Since the question already states 5S is one of the 50S subunit's rRNAs, the other one has to be 23S, not 16S, 18S or 30S which belong elsewhere or don't exist as rRNA species here.
Answer: \[\boxed{23S}\]