To ensure application security during multi-staged login in an application, the developer should ensure that the credentials supplied at each stage are validated before moving to the next stage.

Option A states that the application should validate the credentials supplied at each stage only. This option is partially correct. Validation of credentials at each stage is necessary, but validating only at each stage is not enough. It is important to validate the credentials of previous stages as well to ensure security.

Option B suggests that hidden variables should be used to remember the previous stage values and current stage supplier credentials to be validated at the server end. Hidden variables can be used to store data, but they do not guarantee security. Attackers can access and modify hidden variables, which can lead to security vulnerabilities.

Option C is the correct answer. During multi-staged login, the application should validate the credentials supplied at each stage and previous stages. This ensures that all supplied credentials are correct and valid before allowing access to the application.

Option D suggests that credentials supplied at the previous stage should be saved in a cookie and current stage supplier credentials to be validated at the server end. Saving credentials in cookies can also lead to security vulnerabilities as attackers can access and modify cookies.

Therefore, the correct answer is:

The Answer is: C. During multi-staged login, the application should validate the credentials supplied at each stage and previous stages.