The identity matrix $I_n$ of order $n$ is:
AA diagonal matrix with all diagonal elements equal to 1 and others 0
BA matrix where rows equal columns
CA matrix with all elements equal to 1
DA matrix with all elements 0
Answer & Solution
Correct answer: A. A diagonal matrix with all diagonal elements equal to 1 and others 0
The $n \times n$ identity $I_n$ has $1$s on the main diagonal and $0$s everywhere else. It plays the role of the multiplicative identity: for any compatible $A$, $A \cdot I = I \cdot A = A$.
Option A (all 1s) is the $J$ matrix, sometimes called the all-ones matrix. Option C (all zeros) is the zero matrix, the additive identity.
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