Lambda expressions in Java are primarily used to define the implementation of functional interfaces. A functional interface is an interface that has only one abstract method. Lambda expressions provide a concise way to implement the single abstract method of such interfaces.

The expression 5 > 3 is true because 5 is indeed greater than 3. The other options are not true: 10 != 10 is false (since 10 is equal to 10), (4 + 6) * 2 == 20 is true (since 10 equals 20), and "Java" == "java" is false because string comparison in Java is case-sensitive.

The Runnable interface in Java can be implemented using lambda expressions. It's a functional interface with a single abstract method run(). This allows you to use lambda expressions for tasks that can be executed concurrently. Lambda expressions simplify the creation of anonymous classes for such tasks.

In Java, when you try to override a method from a superclass in a subclass, the method in the superclass must exist; otherwise, it will lead to a compile-time error. Java enforces method signature matching during compile-time, so if the method doesn't exist in the superclass, the compiler will not find a method to override in the subclass, resulting in an error.

The getMessage() method of the Throwable class is used to retrieve the description or message associated with an exception. This message provides additional information about the exception, helping developers understand the cause of the error.

The FileWriter class in Java is used to write characters to a file. It provides methods to write character data to a file in a character stream. FileOutputStream is used for writing binary data, PrintWriter is used for formatted text output, and BufferedReader is used for reading character data, not writing.

a) is correct. If no constructor is provided in a Java class, Java automatically provides a default no-argument constructor. b) is a common misconception; Java only provides a default constructor if you haven't defined any constructors explicitly. c) and d) are incorrect statements.

The lastIndexOf(char ch, int fromIndex) method of the String class is used to find the index of the last occurrence of the specified character within the string, starting the search from the given index fromIndex. The other options are related to the indexOf method but don't perform the same task.

To access or modify private fields in a class, you should use "mutator" methods, also known as setter methods. These methods are designed to set or modify the private field values, maintaining control over access.

Method overriding is a form of polymorphism that occurs at runtime, making it dynamic polymorphism. It allows the actual method to be determined at runtime based on the object's type, which is why it's also known as runtime polymorphism.