To answer this question, we need to understand the purpose and usage of each keyword in the context of instance variables.
A. transient - The transient keyword is used to indicate that a variable should not be serialized when an object is converted into a stream of bytes. This keyword can be used with instance variables.
B. synchronized - The synchronized keyword is used to provide mutual exclusion and ensure thread safety. It is used to control access to shared resources in a multi-threaded environment. This keyword can be used with instance variables.
C. volatile - The volatile keyword is used to indicate that a variable may be modified by multiple threads. It ensures that the variable is always read from and written to the main memory, rather than being cached by individual threads. This keyword can be used with instance variables.
D. abstract - The abstract keyword is used to declare a class or a method as abstract. An abstract class cannot be instantiated, and an abstract method does not have a body and must be implemented by the concrete subclasses. This keyword is not directly related to instance variables and hence, it can be used with them.
Based on the above explanations, the correct answer is B and D. The synchronized and abstract keywords can be used with instance variables, while the transient keyword cannot be used with instance variables.