In high yielding hybrid crop varieties to exploit hybrid vigour, the farmers need to purchase fresh hybrid seed every year because

The correct option is B Hybrid vigour is lost due to inbreeding depression.
Hybrid vigour will be lost due to inbreeding depression. The increase in the level of production traits due to crossing is called hybrid vigour or heterosis. The term hybrid vigour points at the vigour, which is a real factor that can bee seen by the naked eye, in comparison to pure breeding. The other word heterosis is derived from heterozygosity and points toward the direction form where the effect arises. The direction of the effect of crossing is opposite the effect of inbreeding. Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding - ie., breeding of related individuals. Population biological fitness refers to its ability to survive and reproduce itself.