In R81.20, dbedit scripts are being replaced by the mgmt_cli utility for managing and configuring security policies and objects. Here's an explanation of each option:

A. dbedit is not supported in R81.20: This is not entirely accurate. While dbedit is still available and functional in R81.20, it is being phased out in favor of mgmt_cli for policy and object management.

B. dbedit is fully supported in R81.20: This statement is not accurate because although dbedit can still be used, it is not the primary recommended tool for policy management in R81.20.

C. You can use dbedit to modify threat prevention or access policies, but not create or modify layers: This statement is partially true, but it does not provide the complete picture. You can use dbedit for some policy-related tasks, but it's not the primary tool for policy management in R81.20.

D. dbedit scripts are being replaced by mgmt_cli in R81.20: This is the correct and recommended approach. mgmt_cli is the primary tool for managing security policies and objects in R81.20, and it is gradually replacing dbedit for these tasks.

Therefore, option D is the most accurate and recommended answer.

References: Check Point Certified Security Expert (CCSE) R81 documentation and learning resources.