An elementary matrix is a square matrix which differs from the identity matrix of the same order by one single elementary row operation. Consider the matrix given in Option B. M=(
1
5
0
0
1
0
0
0
1
) R1↔R1−5R2 makes it
(
1
5−(5×1)
0
0
1
0
0
0
1
)=(
1
0
0
0
1
0
0
0
1
),
which is an Identity matrix. Thus the matrix M differs from the identity matrix (order 3) by one single row operation. Hence matrix M is an elementary matrix.