The only way to close an SCR is with:

ANS:B - forward breakover voltage

SCR's can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage) , but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device. A uni-junction transistor is commonly used to trigger SCR.