To understand which of the pairs cannot be parallel, let's examine each case:
Step 1: Two diameters of a circle.
Two diameters of a circle are always perpendicular to each other, so they cannot be parallel.
Step 2: Two chords of a circle.
Two chords of a circle can be parallel depending on the position of the chords.
Step 3: A chord and a tangent of a circle.
A chord and a tangent can never be parallel because the tangent touches the circle at only one point and is always perpendicular to the radius at that point.
Step 4: Two tangents of a circle.
Two tangents to a circle at any given point of contact with the circle can be parallel, as they can be at equal distances from the center.
Step 5: Conclusion.
Therefore, the correct answer is (iii) A chord and a tangent of a circle, as these can never be parallel.
Final Answer:} A chord and a tangent of a circle.