Final answer:
Transport of magnesium across the basolateral membrane is likely through facilitated diffusion, as most substances are transported this way according to the information. However, without specific details on magnesium, it's not possible to definitively determine the transport mechanism.
Step-by-step explanation:
Based on the information provided, the transport of magnesium across the basolateral membrane could be facilitated diffusion or secondary active transport. If magnesium movies with the help of a transporter protein, but without direct energy expenditure (ATP), it would be through facilitated diffusion. However, if magnesium is co-transported with another ion, like sodium, which moves down its electrochemical gradient maintained by primary active transport, then it would be secondary active transport. Since the passage mentions that most of the substances transported by a symport mechanism on the apical membrane are transported by facilitated diffusion on the basal membrane, and magnesium diffuses laterally between adjacent cell membranes, facilitated diffusion is likely responsible for the magnesium transport across the basolateral membrane. Yet, without explicit information regarding magnesium specifically, we cannot conclusively determine the exact mechanism of transport.