Solution:
(A) 3 units
Explanation:
From basic ideas of three-dimensional geometry, it is known that $x$-coordinate of a point is its distance from $y z$ plane.
$\therefore$ The distance of Point $P (3, 4, 5)$ from $y z$ plane is given by its $x$ coordinate.
$\because$ $x$-coordinate of point $P = 3$
$\therefore$ The distance of $(3, 4, 5)$ from $y z$ plane is $3$ units
As a result, the corect option is (A).