You are looking for SQL injection vulnerability by sending a special character to web applications. Which of the following is the most useful for quick validation?

Double quotation marks serve as identifier delimiters in ANSI SQL and only act as string delimiters in certain non-standard database configurations, making them unreliable for universal quick validation.

A backslash functions as an escape character in some database-specific contexts but is not a standard SQL string boundary character and does not reliably trigger syntax errors across database platforms.

A semicolon can be used to chain stacked queries in SQL injection but does not immediately break string parsing to produce an observable syntax error the way a single quote does.

SQL uses the single quote as the standard string delimiter across virtually all major database engines including MySQL, MSSQL, Oracle, and PostgreSQL. Injecting a single quote into a parameter causes the database to interpret it as an unmatched string terminator, generating a syntax error that confirms the application is passing unsanitized input directly into SQL queries and is therefore vulnerable to injection.