What access control model below is considered to be the most restrictive access control model, and involves assigning access controls to users strictly according to the custodian?
a) mandatory access control
b) role based access control
c) discretionary access control
d) rule based access control

A) Mandatory Access Control

Explanation:

There are a number of access control models, some of them are as follows :

Mandatory access control: Mandatory access control can be described as, where the user gets access to manage or process some task on the target. It is one of the most restrictive access control models.

Role-based access control: Role-based access control is a type of access control where access can be given on the basis of the particular role, which makes it simpler to process its tasks.

Discretionary Access Control: Discretional access control is a type of access control where access or permission can be given based on the belonging of the group.

Rule-Based Access Control: Rule-based access control is a type of access control where access can be given to the user on a predefined rule made by an administrator.

so accordingly, the most appropriate answer is option A.