The slope of a line m=(y₂-y₁)/(x₂-x₁) ,where (x₁,y₁),(x₂,y₂) are points through which graph passes.

Average rate of reaction at 40 seconds can be determined by determining the slope of line which can be evaluated from the above equation option ii,iii, iv (as the graph passes through these points (50, V₄),(40, V₃),(30, V₂),(20, V₁) respectively) follow the above equation and give the correct and same slope of line.

The Option (i) is false as there is no such (50, V₅) point through which the graph passes.