免费 (miǎn fèi) is the standard, neutral verb for 'free of charge' — used in both speech and writing for anything that costs nothing. 白 (bái) as an adverb means 'free' but is more informal and often implies getting something without paying (e.g., 白吃 'eat for free'). In contrast, 无偿 (wú cháng) is formal and used in legal or official contexts (e.g., 无偿援助 'free aid').