To find the L.C.M. of two terms, take the highest powers of each variable:
- For \( x \), the highest power is \( x^4 \).
- For \( y \), the highest power is \( y^3 \).
- For \( z \), the highest power is \( z^3 \).
Thus, the L.C.M. is:
\[
\text{L.C.M.} = 36x^4 y^3 z^3
\]
Therefore, the correct answer is \( 36x^4 y^3 z^3 \).