If a Daniel cell is connected to an external battery having emf more than that Daniel cell, then

Reverse reaction takes place

The answer is (3). Reverse reaction takes place.

When a Daniel cell is connected to an external battery having an EMF more than that of the Daniel cell, the external battery will force the electrons to flow in the opposite direction. This will cause the reverse reaction to take place, with zinc being reduced and copper being oxidized.

The other options are incorrect.

(1) No reaction takes place. This is not correct, as the reverse reaction will take place.

(2) External EMF has no effect. This is not correct, as the external EMF will force the electrons to flow in the opposite direction, causing the reverse reaction to take place.

(4) All the above. This is not correct, as only the reverse reaction will take place.

Therefore, the only correct option is 3. Reverse reaction takes place.