The Interest Rate Parity (IRP) theory is a key concept in the foreign exchange markets, providing a framework for the relationship between interest rates and the exchange rate. According to the IRP theory, the difference between the spot and forward exchange rates for a currency pair should be equal to the difference between the interest rates of the two countries. This is because any discrepancy between these values would provide an arbitrage opportunity, where traders could borrow in the currency with the lower interest rate, convert to the other currency at the spot rate, lend in the other currency, and then convert back at the forward rate, making a risk-free profit. Therefore, in an efficient market, these arbitrage opportunities should not exist, leading to the interest rate parity condition. This theory is fundamental to the pricing of foreign exchange derivatives, and is widely used in international finance.

Choice A is incorrect. The Purchasing Power Parity theory is not related to the disparity between spot rates and forward rates due to differences in interest rates. Instead, it deals with the concept that the exchange rate between two countries should be equivalent to the ratio of their price levels.

Choice C is incorrect. The Fischer theory, also known as Fisher effect, primarily focuses on nominal interest rates, inflation and real interest rates. It does not explain the relationship between spot and forward rates based on differences in interest rates.

Choice D is incorrect. As explained above, there exists a specific theory – Interest Rate Parity Theory – which explains this relationship between spot and forward exchange rates based on differences in interest rate among countries.