I have a problem. If I sell securities through a sub-broker, which part of the delivery instruction slip should be filled?

Even if you sell securities through a sub-broker, you are required to transfer securities directly to the broker’s pool account only. Hence you need to fill the Market Trade portion of the Delivery Instruction Slip to transfer securities directly to the account of the broker (i.e., the clearing member). Your sub-broker operates on behalf of your broker so the sub-broker will not come in between here in this case.