You have been inducted into a new Clinical Trial Research Product testing team recently. You understand that they have a product that is in production but it has a large number of usability issues that have been recorded against it$.$ This particular product is a data entry product that records information about clinical trials and other associated research information. The primary users of the product are lab research analysts who input up to $5000$ new entries each day. You have been asked to prioritize the usability test cases for one of the user interfaces of the product that can be later applied to other modules. From the list below which of the usability test cases you would create and prioritize in terms of complexity?

What is the correct answer $-$

a$.$ Test Cases to verify the tab$/$screen order of the input fields and screens, to verify proper rule checking for validity for date fields and to check the correctness of the drop down fields

b$.$ Test Cases to verify that the product supports multiple browsers$/$operating systems, the product is secure and data is saved accurately to the database

c$.$ Test cases to verify that access is limited to those with the right permissions, to check if data is saved accurately to the database and to verify the load with virtual users equivalent of $5000$ transactions in an eight hour period

d$.$ Test Cases to verify error handling, if data is not saved to the database, the tab$/$screen order of the input fields and screens and to check the correctness of the drop down fields