Indirect communication between processes P and Q refers to a type of communication where there is an intermediary component involved in the exchange of messages between the two processes. This intermediary component can be a mailbox, which acts as a buffer or storage area for messages.

The correct answer to this question is option C, which states that there is a mailbox to help communication between processes P and Q.

Explanation:
- Indirect Communication: Indirect communication is a form of interprocess communication where processes exchange messages through an intermediary component, such as a mailbox. This is in contrast to direct communication, where processes communicate with each other directly without any intermediary.

- Process P and Q: The question specifically mentions processes P and Q, indicating that these two processes are involved in the communication. Process P wants to send a message to process Q, and vice versa.

- Mailbox: In indirect communication, a mailbox is used as the intermediary component. A mailbox is a data structure or a storage area where messages can be stored and retrieved. It provides a way for processes to communicate with each other indirectly by sending and receiving messages through the mailbox.

- Handling and Passing Messages: The mailbox acts as a buffer or a queue for messages. When process P wants to send a message to process Q, it puts the message into the mailbox. Process Q can then retrieve the message from the mailbox whenever it is ready to receive it.

- Benefits of Mailbox: The use of a mailbox in indirect communication provides several advantages. It decouples the sending and receiving processes, allowing them to operate independently. It also provides a level of indirection, making the communication more flexible and scalable. Additionally, the mailbox can handle situations where the receiving process is not available at the time the message is sent.

- Other Options: The other options mentioned in the question are not correct. Option A states that there is another process R to handle and pass on the messages between P and Q, which is not mentioned in the question. Option B states that there is another machine between the two processes to help communication, which is also not mentioned. Option D states that none of the mentioned options are correct, which is not accurate as option C is the correct answer.

In conclusion, in indirect communication between processes P and Q, a mailbox is used to facilitate the communication by handling and passing on the messages between the processes.