Which of the following drugs is relatively free of drug-drug interactions?

Micafungin is relatively free of drug-drug interactions primarily due to its unique mechanism of action and metabolic pathway. It belongs to the echinocandin class of antifungal agents, which are not significantly metabolized by the liver's cytochrome P450 enzyme system—an area where many other drugs exhibit potential interactions. This characteristic lowers the risk of interaction with other medications that might be metabolized by these enzymes.

In contrast, voriconazole and itraconazole are both azole antifungals that rely heavily on the cytochrome P450 system for their metabolism and can both inhibit or induce various enzymes leading to numerous drug-drug interactions. Terbinafine, while it has a lower interaction potential compared to the azoles, is still subject to some degree of metabolism through the liver, which may lead to drug interactions, although less frequently than voriconazole or itraconazole.

Thus, Micafungin's distinct metabolic pathway allows it to avoid most of the drug-drug interactions that many other antifungal agents are prone to, making it the best answer in this context.