The term used to describe the observed trend in forward prices is 'backwardation'. Backwardation is a situation in the futures market where the forward prices of commodities with distant maturities are lower than the forward prices of the same commodity with shorter maturities. This phenomenon is typically observed when the market expects the spot price of the commodity to decrease over time. In such a scenario, market participants are willing to sell the commodity at a lower price in the future, resulting in a downward sloping forward price curve. This is a common occurrence in the energy sector, particularly in the oil markets, where supply and demand dynamics can significantly influence price trends. Backwardation can provide valuable insights into market expectations about future price movements and can be a useful tool for traders and investors in formulating their trading and investment strategies.

Choice A is incorrect. There is no term such as "Diminishing curve" in the context of futures markets. It seems to be a made-up term and does not describe the observed trend in forward prices.

Choice C is incorrect. The term "Upwardation" does not exist in financial terminology related to futures markets. This choice appears to be a play on words combining 'upward' and 'backwardation', but it's not a recognized concept or phenomenon.

Choice D is incorrect. Contango refers to an upward sloping forward curve, where future prices are higher than spot prices, which is opposite of what Johnson observed in his analysis.