Introduction:
Observing a shadow requires certain conditions to be met. Shadows are formed when light is obstructed by an opaque object and the area behind the object remains unilluminated. In order to observe a shadow, there are certain necessary elements that need to be present. This question asks us to identify which of the given options is not always necessary to observe a shadow.

Explanation:
To understand why option 'A' (Sun) is not always necessary to observe a shadow, let's break down the other options and their importance in forming a shadow.

1. Source of Light:
A source of light is essential to create a shadow. Light can come from various sources such as the sun, a lamp, or a flashlight. Without a source of light, there would be no illumination, and therefore, no shadow. So, a source of light is always necessary to observe a shadow.

2. Opaque Object:
An opaque object is required to block the path of light and create the shadow. Opaque objects do not allow light to pass through them, unlike transparent or translucent objects. When light encounters an opaque object, it cannot pass through, and as a result, a shadow is formed. Therefore, an opaque object is always necessary to observe a shadow.

3. Screen:
A screen is not always necessary to observe a shadow. While a screen can be used to project and capture the shadow, it is not a prerequisite for the formation of a shadow. Shadows can be observed without the presence of a screen by simply looking at the area where the light is blocked by the opaque object.

4. Sun:
Contrary to the other options, the sun is not always necessary to observe a shadow. Although the sun is a common and powerful source of light, shadows can be formed with other sources of light as well, such as artificial lights or even a flashlight. Therefore, the presence of the sun is not an absolute requirement to observe a shadow.

In conclusion, while a source of light and an opaque object are always necessary to observe a shadow, the presence of a screen and specifically the sun are not always essential. Shadows can be observed without the use of a screen, and alternative sources of light can be used in place of the sun.