According to Oracle's documentation12, ASM failure groups provide redundancy by storing mirror copies of data on different disks or storage servers. When ASM allocates an extent for a normal redundancy file, ASM allocates a primary copy and a secondary copy. ASM chooses the disk on which to store the secondary copy so that it is in a different failure group than the primary copy1. This way, if one disk or storage server fails, ASM can still access the data from another failure group2.
Therefore, the statement that is true about how ASM failure groups provide redundancy is:
Failure groups contain all ASM disks in a single storage server preventing mirror copies being written to the same storage server.