I think the you have written the formula wrong . In efficiency for the sliding window protocol we follow the formula n = N/(1+2a) where a = Tp/ Tt. Where N = The sliding window size in the sending side.
b) Now as RTT increases propagation time also increases , so the a increases and efficiency(n) decreases. We know Throughput = Efiiciency * Bandwidth. So as Efficiency decreases, Throughput also decreases.
c) As the window size(N) decreases so generally efficiency also decreases. Hence Throughput decreases.