**Explanation:**

- In physics, dimensional analysis is a powerful tool used to check the consistency of equations and to derive relationships between physical quantities. It involves examining the dimensions (units) of the quantities involved in a problem to determine if they are compatible.

- The dimensions of a physical quantity are the powers to which the fundamental units are raised in order to express that quantity. For example, the dimensions of length are [L], the dimensions of time are [T], and the dimensions of mass are [M].

- In the given options, we need to identify the set of quantities that have different dimensions.

**a) Pressure, Young's modulus, Stress**

- Pressure is defined as force per unit area and its dimensions are [ML^-1T^-2].
- Young's modulus is a measure of the stiffness of a material and its dimensions are [ML^-1T^-2].
- Stress is defined as force per unit area and its dimensions are [ML^-1T^-2].
- Since all the quantities have the same dimensions, this set does not have different dimensions.

**b) Emf, Potential difference, Electric potential**

- Emf (electromotive force) is the energy per unit charge provided by a source of electric energy and its dimensions are [ML^2T^-3I^-1].
- Potential difference is the work done per unit charge in moving a charge between two points and its dimensions are [ML^2T^-3I^-1].
- Electric potential is the electric potential energy per unit charge and its dimensions are [ML^2T^-3I^-1].
- Since all the quantities have the same dimensions, this set does not have different dimensions.

**c) Heat, Work done, Energy**

- Heat is the transfer of energy between two objects due to a temperature difference and its dimensions are [ML^2T^-2].
- Work done is the product of the force applied to an object and the displacement of the object in the direction of the force and its dimensions are [ML^2T^-2].
- Energy is the ability to do work and its dimensions are [ML^2T^-2].
- Since all the quantities have the same dimensions, this set does not have different dimensions.

**d) Dipole moment, Electric flux, Electric field**

- Dipole moment is a measure of the separation of positive and negative charges in a system and its dimensions are [Q.L].
- Electric flux is the measure of the electric field passing through a surface and its dimensions are [Q].
- Electric field is the force experienced by a unit positive charge placed in an electric field and its dimensions are [ML^3T^-3Q^-1].
- Since the dimensions of dipole moment, electric flux, and electric field are different, this set has different dimensions.

Therefore, the correct answer is option 'D'.

D