The correct answer to the analogy "Square : Diamond : : Circle : ?" is option C) Oval.

Explanation:
In this analogy, we need to find a relationship between the first pair of words (Square and Diamond) and then apply the same relationship to the second pair of words (Circle and ?).

1. Relationship between Square and Diamond:
- A square is a four-sided polygon with equal sides and angles, while a diamond is also a four-sided polygon, but with unequal sides and angles.
- The key difference between a square and a diamond is that a square has equal sides and angles, while a diamond does not.

2. Applying the same relationship to Circle and the missing word:
- A circle is a two-dimensional geometric shape with all points equidistant from the center. It has no sides or angles.
- To find the word that shares a similar relationship with a circle, we need to look for a shape that is also two-dimensional, has no sides or angles, and has all points equidistant from the center.

3. Identifying the correct answer:
- Among the given options, option C) Oval fits the criteria for the missing word.
- An oval is a two-dimensional shape that resembles a squashed or elongated circle. It has no sides or angles, and all points on the oval are equidistant from the center, just like a circle.

Therefore, the correct answer is option C) Oval.

Note: The other options - a) Smooth, b) Round, and d) Ball - do not fit the analogy because they do not share the same relationship with a circle as a diamond does with a square.