The correct sequence of the topographical features found from the upper to the lower course of a river is option 3: Rapids - Ox-bow lake - Estuary.

Rapids are found in the upper course of a river. They are fast-flowing, turbulent sections of the river where the gradient is steep. The water rushes over rocks and creates a tumultuous flow.

Downstream from the rapids, in the middle to lower course of the river, we find ox-bow lakes. Ox-bow lakes are formed when a meander, a bend in the river, is cut off from the main channel. Over time, the meander becomes isolated, forming a curved lake.

Towards the end of a river`s course, it empties into an estuary. An estuary is a semi-enclosed coastal body of water where freshwater from the river meets and mixes with saltwater from the ocean. It is characterized by brackish water, which is a mix of freshwater and saltwater.

Therefore, the correct sequence is Rapids - Ox-bow lake - Estuary. Alert - correct answer should be option 4: Estuary - Ox-bow lake - Rapids.