The convective acceleration in a flow field refers to the change in velocity of a fluid particle as it moves along a streamline. It is caused by the change in velocity magnitude and direction along the streamline. In this question, we are asked to identify which type of flow field has only convective acceleration.

A steady uniform flow is a flow field in which the velocity magnitude and direction are constant at every point in the fluid at any given time. In this case, there is no change in velocity along the streamline, and therefore no convective acceleration. Hence, option 'A' is not the correct answer.

An unsteady uniform flow is a flow field in which the velocity magnitude and direction change with time, but the change is uniform throughout the fluid at any given time. Similar to a steady uniform flow, there is no change in velocity along the streamline, and hence no convective acceleration. Therefore, option 'B' is not the correct answer.

A steady non-uniform flow is a flow field in which the velocity magnitude and direction vary at different points in the fluid, but they do not change with time. In this case, there is a change in velocity along the streamline, resulting in convective acceleration. Therefore, option 'C' is the correct answer.

An unsteady non-uniform flow is a flow field in which the velocity magnitude and direction vary both with time and at different points in the fluid. This type of flow field will have convective acceleration due to both the changes in velocity magnitude and direction along the streamline. Therefore, option 'D' is not the correct answer.

In summary, a flow field that has only convective acceleration is a steady non-uniform flow where the velocity magnitude and direction vary at different points in the fluid, but they do not change with time.