C

To analyze the effects of the four situations on the concave mirror:

1. **A part of the mirror is removed**: This will not necessarily change the intensity of the image but might affect the overall reflected light, so this doesn’t correlate perfectly with [A], which focuses on intensity.

2. **A part of the mirror is covered with black paper**: This will block light from that segment, reducing the overall reflected light to the focal point, which may connect with [B] (although this pairing is also about intensity). However, due to the presence of the source light elsewhere, the position will not necessarily change since it depends on where the source is located.

3. **The mirror is rotated by a small angle θ**: Rotating the mirror will change the path of the reflected rays while keeping the same focal length. This may change the size and possibly the orientation of the image, aligning correctly with [C], which states that the size of the image may change.

4. **The mirror is displaced keeping the principal axis parallel**: Displacement of the mirror while keeping the axis parallel does not actually change the focal length, so this does not align with the claim in [D]. The focal length of the mirror remains the same unless the curvature or material changes.

Thus, the correct answer for the pairing of (iii) is [C].