(a) Both (A) and (R) are true and (R) is the correct explanation for (A).
The term "resolution" refers to the number of pixels in an image, expressed as width × height (e.g., 1280×1024), so (R) correctly explains (A).
Source: Chapter 5, Section 5.2 – Resolution
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The textbook explicitly states: "The number of pixels in an image is sometimes called the resolution… one convention is to express resolution as the width by the height, for example, 1280×1024." Both the Assertion and Reason are accurate, and the Reason directly defines/explains the Assertion — so option (a) is correct. Note: the Reason says "height × width" while the book says "width × height" — both express the same idea and are acceptable.