The best choice for a drive that will be used with both Windows 10 and macOS is:
B. FAT32
FAT32 is compatible with both operating systems, allowing you to read and write files without issue. However, it has a file size limit of 4GB, which may be a constraint if you plan to store larger files.
While you might consider D. NTFS (which is the default file system for Windows), macOS can only read NTFS drives natively; writing to them requires third-party software. C. HFS is primarily for macOS and not readable by Windows without third-party software. A. Ext4 is mainly used in Linux and is not natively supported by either Windows or macOS.
If the 4GB file size limit in FAT32 is a concern, you might also consider using exFAT, which is compatible with both Windows and macOS and does not have the same file size limitations as FAT32. However, exFAT is not listed as an option in your query.