Final answer:
The subject being discussed is ribosomal proteins, which are components of ribosomes that assist in the process of protein synthesis. The size of these proteins varies, and there is one protein, S20/L26, which is common to both the small and large subunits of the ribosome.
Step-by-step explanation:
The correct answer to the question is A. Ribosomal proteins. These proteins range in size and are part of the structure of ribosomes. Ribosomes are complex macromolecules that play a critical role in protein synthesis. They are composed of ribosomal RNA (rRNA) and ribosomal proteins, and they assemble amino acids into proteins during translation. The statement 'S20 = L26' refers to the ribosomal protein S20 in the small subunit being equivalent to L26 in the large subunit. With this exception, a single protein is found in both the small and large ribosomal subunits.
Ribosomes have two different subunits: a small one responsible for binding the mRNA template and a large one responsible for sequentially binding tRNAs. The sizes of ribosomal subunits are designated in Svedberg units, with prokaryotic ribosomes typically being 70S (30S small subunit and 50S large subunit) and eukaryotic ribosomes being 80S (40S small subunit and 60S large subunit). These subunits come together during the initiation of translation when the ribosome starts synthesizing proteins.