The marginal rate of substitution of X for Y at a point on an indifference curve equals:

1. Along an indifference curve the utility lost from less Y equals the utility gained from more X. 2. This gives $MRS_{xy} = \frac{MU_x}{MU_y}$, the slope of the indifference curve. 3. The inverted ratio, the price-MU mix, and the product form are all incorrect, so those options are wrong. _Source: ICAI BoS CA Foundation Paper 4 Business Economics, Ch 2 Unit II "Theory of Consumer Behaviour", p.16_