Cash flow management is concerned with managing the movement of cash: accelerating collections and controlling disbursements so that cash is available when needed. The techniques listed should be judged one by one.

Lock-box system: customers send payments to a post office box near them, which a local bank empties several times a day and credits directly to the firm's account. It cuts mailing and processing time, so it speeds up collections and is a cash flow management tool.

Playing the float: the firm uses the difference between the balance in its own books and the balance in the bank's books (disbursement float) by timing payments so that cheques issued are cleared later. It is a disbursement control technique and is part of cash flow management.

Concentration Bank Account: collections received at many local branches are transferred to one central (concentration) bank account, reducing idle balances and giving centralised control. This is also a collection and cash flow technique.

Marketable securities relate to the separate decision of investing surplus or idle cash in near-cash assets such as treasury bills and commercial paper. They belong to the management of cash balances and liquidity rather than to the techniques of managing the flow of cash, so they are left out of this combination.

Hence, I, III and IV only.