A Silicon Controlled Rectifier (SCR) is made up of silicon and not germanium because

The correct answer is option 3
Concept:
An SCR is made of silicon because Silicon has a smaller leakage current than Germanium.

A Silicon Controlled Rectifier (SCR) is a 3-terminal and 4-layer semiconductor current-controlling device. It is mainly used in devices for the control of high power.

SCR has three operating modes.

Forward blocking mode:

• Anode voltage is made positive with respect to the cathode, with the gate circuit open
• Both outer junctions are forward biased
• The inner junction is reversed and biased
• SCR remains in the off state

Forward conduction mode:

• The anode voltage is greater than the forward break-over voltage
• All three junction is forward biased
• The device is in conduction mode

Reverse blocking mode:

• The cathode is at a higher potential than the anode
• Both outer junctions are reverse-biased (high resistance)
• The inner (middle) junction is forward-biased. 
• It is reversed biased and non-conducting

The forward and reverse characteristic of an SCR is as shown: