Correct is A C E
A. They support up to a max of 100 of columns
C. They avoid resource intensive joins
E. They do not include soft-deleted records

https://developer.salesforce.com/docs/atlas.en-us.salesforce_large_data_volumes_bp.meta/salesforce_large_data_volumes_bp/ldv_deployments_infrastructure_skinny_tables.htm

B can't be true, because if you modify a source object, the related skinny table becomes invalid. The answer is ACE
C because a primary purpose of a skinny table is to avoid a join between a table containing standard fields and another table containing custom fields.

A,B,E looks correct. Refer : https://salesforcelightningblog.wordpress.com/2018/04/04/skinny-tables-in-salesforce/