If the load doubled in value in the given circuit, what reflected resistance would the source see?

ANS:A - 80

When referred to primary side, the secondary rest. R2 becomes R2'=R2/(K^2) where K=N2/N1.
i.e. Transformation ratio so using the equation, we get R2'=1000/(5/1)^2 =80 ohms.
For referring primary rest, to the secondary side, use formula R1'=(K^2)*R1.
Transformation ratio(K)= N2/N1,
Turns ratio(a)= N1/N2,
So, Transformation ratio= 1/Turns ratio.