Correct Answer - Option 3 : Primary Star, Secondary Delta
The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.
For differential protection of power transformer, CT s must be connected in star in the delta side of the transformer and in delta in the star side of the transformer.
The given connection for the transformer is Δ-Y.
The star side must be connected in delta. Therefore, the connection for the CT must be Y-Δ connection.
Therefore, primary of CT must be star and the secondary must be delta.