D

In an induction coil, when the circuit is broken, the change in current induces a high electromotive force (e.m.f.) in the secondary winding due to the principle of electromagnetic induction, specifically Faraday's law of induction.

According to Faraday's law, the induced e.m.f. is directly proportional to the rate of change of magnetic flux. At the moment the circuit is broken, the current changes from a certain value to zero almost instantaneously, which creates a large change in magnetic flux and thus induces a high e.m.f. in the secondary coil. Conversely, when the circuit is made (or closed), the change in current is not abrupt, and hence the induced e.m.f. is not very high.

Therefore, the correct answer is Option D: Very high during break of the circuit.