A Query Activity in an Automation can fail for several reasons, including:

The query takes more than 60 minutes to run (A) - Long-running queries are subject to timeouts, which can cause the query to fail.

The query results in duplicate rows not allowed by the primary key (C) - If the query attempts to insert duplicate rows into a data extension that has a primary key constraint, it will fail.

The query is inserting a value that is larger than the size of a field (E) - If the query attempts to insert data that exceeds the field size limits, it will result in an error.