To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) transient - This option is incorrect because the "transient" keyword can be used with instance variables. It is used to indicate that a variable should not be serialized when an object is being converted to a byte stream.
Option B) synchronized - This option is correct because the "synchronized" keyword cannot be used with instance variables. It is used to provide mutual exclusion and synchronization between multiple threads, but it can only be used with methods or code blocks.
Option C) volatile - This option is incorrect because the "volatile" keyword can be used with instance variables. It is used to indicate that a variable may be modified by multiple threads and to ensure that the most up-to-date value of the variable is always read.
Option D) abstract - This option is correct because the "abstract" keyword cannot be used with instance variables. It is used to declare abstract classes or methods, but it is not applicable to variables.
The correct answer is B, D. The "synchronized" and "abstract" keywords cannot be used with instance variables.