When the correlation between interest rates and futures prices is positive, futures contracts are more desirable to holders of long positions than forward contracts. This is because rising prices will lead to futures profits that are reinvested in periods of rising interest rates and falling prices will lead to losses that occur during periods of falling interest rates. Therefore, it is far better to receive cash flows in the interim than the expiration under such conditions. This is due to the fact that futures contracts are marked to market daily, meaning that gains and losses are realized and can be reinvested daily. This allows the holder of a long futures contract to potentially earn interest on their gains, which can be particularly beneficial in a rising interest rate environment.

Choice A is incorrect. A zero correlation between futures prices and interest rates would not make a long position in futures contracts more desirable than forward contracts. This is because the value of a futures contract is not influenced by changes in interest rates when there's no correlation.

Choice C is incorrect. Negative correlation between futures prices and interest rates would make a long position in forward contracts more desirable than futures contracts, not the other way around. This is because as interest rates decrease (increase), the price of future contracts also increases (decreases).

Choice D is incorrect. Knowing the correlation in advance using the spot curve does not necessarily make a long position in futures more desirable than forwards. The desirability depends on whether this known correlation is positive or negative, rather than just knowing it.