Bleeding is the correct answer in this case. Let's understand why this is the correct option and why the other options are incorrect.

1. Segregation:
- Segregation refers to the separation of coarse aggregates from the concrete mix during transportation or pouring.
- It occurs when the mix is not properly proportioned or when excessive water is added to the mix.
- Segregation can cause uneven distribution of aggregates, resulting in a non-uniform concrete mix and poor finish.
- However, it does not directly affect the smoothness of the finish.

2. Internal Friction:
- Internal friction occurs due to the interaction between different particles in the concrete mix.
- It can affect the workability of the mix, making it more difficult to place and finish.
- However, it does not specifically cause difficulty in obtaining a smooth finish.

3. Hardness:
- Hardness refers to the strength and durability of the concrete mix.
- While a hard mix may be challenging to work with, it does not directly impact the smoothness of the finish.
- The smoothness of the finish is primarily affected by the workability and consistency of the mix.

4. Bleeding:
- Bleeding refers to the process where water from the concrete mix rises to the surface and forms a layer of water on top of the freshly poured concrete.
- It occurs when the water content in the mix is too high or when the mix is not properly consolidated.
- Bleeding can cause several issues, including difficulty in obtaining a smooth finish.
- The layer of water on the surface can interfere with the finishing process, making it challenging to achieve a smooth and uniform surface.

Therefore, the correct answer is option D - bleeding. Bleeding in the concrete mix can lead to difficulty in obtaining a smooth finish due to the presence of a layer of water on the surface of the freshly poured concrete.