UPSC NDA I 2015 Mathematics Solved Paper

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 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.