Given:
Balance as per pass book = Rs. 20,000
Direct deposit by customer = Rs. 4,000

To find:
Balance as per cash book

Solution:
The cash book is a record of all transactions related to cash inflows and outflows. It includes all transactions related to cash, bank, and discount. The passbook is a copy of the customer's account in the bank's books, which shows all transactions done by the customer in the bank.

To find the balance as per cash book, we need to consider the following:

1. Direct deposit by customer:
When a customer directly deposits money into the bank account, it will not be recorded in the cash book until the bank receives the information. Therefore, the balance as per cash book will be less than the balance as per passbook until the bank updates the account.

In this case, the customer deposited Rs. 4,000 directly into the bank, which is not recorded in the cash book. Therefore, the balance as per cash book will be Rs. 20,000.

2. Bank charges and interest:
The bank may charge some fees or interest on the account, which will be recorded in the passbook. However, it may not be recorded in the cash book until the bank receives the information. Therefore, the balance as per cash book may differ from the balance as per passbook.

3. Cheques issued and deposited:
If a cheque is issued by the customer, it will be recorded in the cash book as an outflow. If a cheque is deposited, it will be recorded as an inflow. However, it may take some time for the cheque to clear, and the amount will be updated in the passbook only after clearance. Therefore, the balance as per cash book may differ from the balance as per passbook until the cheque is cleared.

In this case, we do not have any information about bank charges, interest or cheques issued/deposited. Therefore, we can assume that there are no such transactions.

Therefore, the balance as per cash book = balance as per passbook - direct deposit by customer
= Rs. 20,000 - Rs. 4,000
= Rs. 16,000

Hence, the correct answer is option C - Rs. 16,000.