Option 1 states that both the assertion (A) and the reason (R) are true, and R is the correct explanation of A. This means that constant dredging is required in the Kolkata Port, and the reason for this is the high rate of silting in the Hooghly River.

To understand this, we need to know what dredging is and why it would be necessary in a port. Dredging is the process of removing sediment, silt, and other debris from the bottom of a body of water to maintain or increase water depth. Ports often require dredging to remove silt and maintain navigability for ships.

The assertion (A) states that constant dredging is required in the Kolkata Port. This means that ongoing dredging activities are necessary for the port to function effectively.

The reason (R) explains why constant dredging is required in Kolkata Port by stating that the rate of silting in the Hooghly River is very high. Silting refers to the accumulation of fine sediment and silt in a river. The high rate of silting in the Hooghly River would necessitate regular dredging to keep the port accessible for large vessels.

Therefore, option 1 is the correct answer as both the assertion and reason are true