A row matrix is a matrix with only one row. The order of a matrix is given as (number of rows) x (number of columns).
We need to identify which of the given options cannot represent the order of a row matrix.
Option (A) $2 \times 1$ represents a matrix with 2 rows and 1 column. This is a column matrix, not a row matrix.
Option (B) $1 \times 2$ represents a matrix with 1 row and 2 columns. This is a row matrix.
Option (C) $1 \times 1$ represents a matrix with 1 row and 1 column. This is a row matrix.
Option (D) $1 \times n$ represents a matrix with 1 row and $n$ columns, where $n$ is any positive integer. This is a row matrix.
From the above analysis, only option (A) $2 \times 1$ does not represent a row matrix.
Final Answer: $2 \times 1$
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