The MRTS (Marginal Rate of Technical Substitution) on an isoquant is defined as MPK/MPL at the relevant point on the isoquant. Hence the correct option is a.
The Marginal Rate of Technical Substitution (MRTS) measures the rate at which one input can be substituted for another in the production process while maintaining the same level of output. On an isoquant, which represents various combinations of inputs that produce the same level of output, the MRTS is given by the ratio of the marginal product of capital (MPK) to the marginal product of labor (MPL) at the relevant point on the isoquant.
This is expressed mathematically as MRTS = MPK/MPL. The MRTS indicates the trade-off between inputs and is crucial in optimizing production processes by guiding decisions on how to allocate resources efficiently while maintaining a constant level of output. Therefore, option MPK/MPL accurately represents the definition of MRTS on an isoquant. Hence the correct option is a.