To answer this question, the user needs to know about non-functional software testing and the different types of software testing.

Usability testing is a non-functional software testing that is performed to check if the user interface is easy to use and understand, making option A the correct answer.

Option B, Security testing, is a type of functional software testing that checks if the software system is secure from external threats and internal vulnerabilities.

Option C, Unit testing, is a type of functional software testing that checks if the individual units or components of the software system are working as intended.

Option D, Black Box testing, is a type of functional software testing that checks if the software system is working as intended by testing it without any knowledge of its internal workings.

Therefore, the correct answer is:

The Answer is: A. Usability Testing