Identify the security mistakes that the developer has coded?

Sam, an application security engineer working in INFRA INC., was conducting a secure code review on an application developed in Java. He found that the developer has used a piece of code as shown in the following screenshot.

Identify the security mistakes that the developer has coded?
A . He is attempting to use client-side validation
B. He is attempting to use whitelist input validation approach
C. He is attempting to use regular expression for validation
D. He is attempting to use blacklist input validation approach

Answer: D