The correct option is (A) 7 years.
Literacy Rate measures the proportion of literate population in the 7 years and above age group.
Source: Income and Other Criteria, Chapter 1
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The textbook explicitly defines Literacy Rate in Table 1.4's footnote: "Literacy Rate measures the proportion of literate population in the 7-and-above age group." This is a standard definition used by the Census of India. Remember this exact age (7 years) — examiners often test this as a fill-in-the-blank or MCQ.