What determines whether a protein will be produced on a "free ribosome" or on one attached to the endoplasmic reticulum?

A) a chemical signal on the protein being produced
B) a special sequence of nucleotides on the mRNA molecule
C) a special sequence of amino acids on the protein being produced
D) whether the mRNA first binds to a "free ribosome" or one attached to the endoplasmic reticulum