Below are matrices.

A= 4 5 2 1

0 1 8 2

0 15 2 1

** A is a 3x4 matrix.

L = 1 0 0

0 1 0

0 k 1

A = LU

If U is found by exactly 1 row-operation to A, what is the value of k?

I do not think that your question makes sense. What is U?