Explanation
A leading wildcard is a wildcard that is placed at the beginning of a search term, such as * or ?. A leading wildcard matches any characters that precede the specified term. For example, filemod:/system32/ntdll.dll matches any file modification events that end with /system32/ntdll.dll, regardless of the drive letter or the directory name. A leading wildcard is not recommended unless absolutely necessary because it carries a significant performance penalty for the search. This is because the search engine has to scan the entire index for possible matches, rather than using the index to quickly narrow down the results1.
The other options are not examples of leading wildcards. A. filemod:system32/ntdll.dll is an exact match query that matches only file modification events that are exactly system32/ntdll.dll. B. filemod:system32/ ntdll.dll is a trailing wildcard query that matches any file modification events that start with system32/ and end with ntdll.dll, regardless of the characters in between. D. filemod:system32/ntdll.dll is a trailing wildcard query that matches any file modification events that start with system32/ntdll.dll, regardless of the characters that follow. References:
Search Syntax - VMware Docs, Wildcards section.