Actually here we can see that the inverters are cascaded i.e. placed once after the another..Hence the propogation delay of each gate adds up..
So overall delay = 5 * 100 ps
= 500 ps
Hence fundamental frequency which drives the oscillator = 1 / delay
= 1 / (500 * 10-12) Hz
= 1000 / 500 GHz
= 2 GHz
Hence D) is the correct answer..