To drop all celldisks on all the storage servers in an X9M-2 quarter rack, you should use the CELLADMIN account, which has the necessary privileges to perform this task. You can use the CELLCLI command-line interface to drop the celldisks. The best way to do this is by calling CELLCLI on all cells using DCLI (Oracle Database Command Line Interface) which allows you to run commands on multiple servers at once. Alternatively, you can use an administrator-created storage server user with appropriate privileges on celldisk objects by calling CELLCLI on all cells using exadcli. It is not recommended to use the CELLMONITOR account, as it has a more limited set of privileges. It is also important to note that EXACLI is not a valid tool for this task

In order to power off an Exadata Database Machine in an orderly fashion, the commands should be executed in the following order: 1) Execute "crscti stop cluster -all" as the grid user from one database server; 2) Execute "shutdown -h now" on all database servers; 3) Power off all network switches; 4) Execute "shutdown -h now" on all Exadata storage servers; and 5) Power off the rack using the power switches on the PDUs. This sequence of commands ensures that all services running on the Exadata Database Machine are in a clean state before powering off the system. (Source: Oracle Exadata Database Machine X9M Implementation Essentials, page 543)

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.

1. ASR can upload configuration metadata to support problem resolution1.
2. ASR Manager opens a service request (SR) automatically after sensors detect hardware faults1
https://docs.oracle.com/cd/E37710_01/doc.41/e23333/toc.htm