To answer this question, the user needs to know what an SQL debug message is and how it can be exploited by an attacker.

An SQL debug message is a type of error message that can occur when there is an issue with an SQL query on a website or application. These messages can contain sensitive information about the query being executed, such as the database name, table name, and even the query itself.

Now, let's go through each option and explain why it is right or wrong:

A. steal sensitive information from other users: This option is possible because if an attacker can access the debug message, they can potentially see sensitive information about the database and query being executed. This information can be used to steal data from other users.

B. run scripts on other user's browser: This option is not directly related to an SQL debug message. It is more related to cross-site scripting (XSS) attacks, which are a different type of vulnerability.

C. alter the communication protocol used by the site: This option is not directly related to an SQL debug message. It is more related to protocol-level attacks, which are a different type of vulnerability.

D. can potentially understand the query's structure: This option is correct. An attacker who can access the SQL debug message can potentially understand the query's structure, which can be used to craft more effective SQL injection attacks.

Therefore, the correct answer is:

The Answer is: D