The correct answer is D) heavy coil springs. The safety heads of most large reciprocating compressors used in refrigeration systems are held in place by heavy coil springs. This is a standard safety feature designed to prevent the safety head from being blown off the compressor in the event of a sudden pressure increase, which could otherwise cause a dangerous explosion. The coil springs are sized to withstand the maximum expected discharge pressure while still allowing the safety head to lift and relieve pressure if needed. The other answer choices are incorrect because: A) Teflon gaskets would not be strong enough to securely hold the safety head in place, B) tack welding would make the safety head difficult to inspect and replace, and C) the discharge pressure in the relief valve return line does not directly hold the safety head in place.