Standard mail: Google will deliver a postcard to the address of your business with a 5-digit code into the appropriate field.
Email: An email will be sent to your business email address with a link for you to enter 5-digit code.
Phone: you have the option to get a call or automated text to your phone with a 5-digit code.
Unfortunately, verifying your business with GMB is notoriously tricky, as some verification options might not be available. Many business owners even cite that it takes several times to verify their business or issues with bulk verifications if they operate franchises in different locations. However, despite sometimes being a difficult process, the positives of having a GMB listing far outweigh the initial hassle.