I happen to differ from Arjun Sir.
Please let me know my faults, if any.
Taking for write through
T(avg read)= Hit ratio (read)* access time of CM+ Miss ratio (read)*(Read allocate time+ access time of CM)
And,
T(avg write)= Hit ratio (write)*write through time + miss ratio (write)*(write allocate+write through time)
Because write/read allocate as given in the question is to get 2 word block to be transferred from MM to CM. Taking read/write allocate as 2*100ns
And write through time is max (Tm, Tc)= 100ns
So
T read= 0.8*20+0.2(100*2+20)
=60ns
T write= 0.9*100+0.1(100*2+100)
=120ns
Tavg= 0.7*60+0.3*120
=78ns
Tavg is time for 1 word access
Therefore, in one second
1000/78 million words will be accessed on average.
This evaluates to 12.8 million words/second.
Hence, option b.