The search can be rewritten to maximize efficiency by using the index option. The index option is used to specify the index to search. This option is useful when you have multiple indexes and want to search only one of them. The index option is also useful when you want to search a specific index that is not the default index. The index option can reduce the search time and resource consumption by limiting the scope of the search. Therefore, the correct answer is C. Option C, which uses the index option to search only the main index for the sourcetype web_access. References:

Splunk Core Certified Consultant Test Blueprint

Splunk Documentation: Specify indexes in your search

Splunk Documentation: Use the index option