To solve this question, the user needs to understand the purpose of secondary indexes and their impact on the database system.

Secondary indexes are additional indexes created on a table to improve the performance of database queries. They allow queries to search for data using columns other than the primary key, which can speed up query execution times. However, secondary indexes can also have a negative impact on the database in terms of disk space consumption and performance overhead.

Option A is incorrect because the selection of an optimal query execution method is not affected by the creation of secondary indexes. Query execution methods are determined by the database management system's query optimizer, which decides the best way to execute a query based on factors such as table statistics, query complexity, and available resources.

Option B is correct because secondary indexes require additional disk space to store the index data. This can be a significant issue for large databases with many indexes, as it can lead to increased storage costs and slower query performance due to disk I/O.

Option C is correct because secondary indexes can also lead to performance overhead when performing database operations such as insert, update, and delete. When these operations are performed, the database must update the primary index as well as any secondary indexes that reference the affected columns. This can lead to slower query execution times and increased system resource usage.

Option D is incorrect because the creation of secondary indexes does not have a direct impact on database security. However, it is important to ensure that only authorized users have access to create, modify, or delete indexes to maintain the security of the database.

Therefore, the correct answer is:

The Answer is: E