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The identity matrix $I_n$ is a square matrix in which:
AAll entries are $1$
BAll entries are $0$
CDiagonal entries are $1$ and all off-diagonal entries are $0$
DDiagonal entries are $0$ and all off-diagonal entries are $1$
Answer & Solution
Correct answer: C. Diagonal entries are $1$ and all off-diagonal entries are $0$
$I_n$ has $a_{ii} = 1$ for $i = 1, \dots, n$ and $a_{ij} = 0$ for $i \ne j$. Its key property is $AI = IA = A$ for any compatible matrix $A$. A matrix with all entries $1$ is a 'matrix of ones', not the identity.
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