Answer:
Using put call parity:
C + X/(1+r)^n = S+P
C + 18/(1+0.08)^1 = 20+3.33
C + 18/1.08 = 20 + 3.33
C + 18/1.08 = 23.33
C + 16.67 = 23.33
C = 23.33 - 16.6667
C = 6.67
The call price ($7) is over price, so we should sell call and buy underlying ($6.67). After one year, the underlying option will get a gain of $0.33 ($7-$6.67). So, we should exploit this arbitrage opportunity.