E was correct at the time of this question but on Spring '24 they changed it to 200
C is incorrect

C is a bit confusing. Whether it is talking about data or metadata? It data then skinny table is in sync with source table, but if it is talking about metadata then skinny table will have to be recreated.

A B C

Salesforce can create skinny tables to contain frequently used fields and to avoid joins. This can improve the performance of certain read-only operations. Skinny tables are kept in sync with their source tables when the source tables are modified.

For each object table that’s visible to you, Salesforce maintains other, separate tables at the database level for standard and custom fields. This separation, which is invisible to customers, ordinarily requires a join when a query contains both kinds of fields. A skinny table contains both kinds of fields and also omits soft-deleted records.

Skinny tables can contain a maximum of 200 columns.
Skinny tables can’t contain fields from other objects.

ABE correct ans
A. They do not include soft-deleted records
B. They avoid resource intensive joins
E. They support up to a max of 100 of columns

C is not correct in this context and they are not synced if the table is modified (a change in field), they become invalid and SF needs to be contacted again to have the skinny table created

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