Step 1: Do the operation.
New first row: $(-5 + 3 \cdot 2,\ -3 + 3 \cdot 1) = (1, 0)$.
Step 2: Write the matrix.
\[ \begin{pmatrix} 1 & 0 \\ 2 & 1 \end{pmatrix} \]
Step 3: Test each option.
Upper triangular needs the bottom-left entry to be 0, but it is 2, so (A) is out. Identity needs 0 there, so (C) is out. Determinant $= 1 - 0 = 1 \neq 0$, so (D) is out. The top-right entry is 0, so it is lower triangular.
Final Answer:
Option (B).
\[ \boxed{\text{Lower triangular matrix}} \]