Horizontal tangents are such that d<em>y</em>/d<em>x</em> = 0; this happens when
(<em>y</em> - 3) / (<em>x</em> - 3) = 0 → <em>y</em> - 3 = 0 → <em>y</em> = 3
so (I) is true.
Vertical tangents are such that their slope is undefined; this happens when the denominator vanishes, or
<em>x</em> - 3 = 0 → <em>x</em> = 3
so you are right that (II) is not true.
To assess whether (III) is correct, consider what happens at different nearby points (with neither <em>x</em> = 3 nor <em>y</em> = 3) where the <em>y</em> coordinate is kept the same so that the tangent lines occur in the same row. For example,
<em>x</em> = 0, <em>y</em> = 0 → d<em>y</em>/d<em>x</em> = (-3)/(-3) = 1
<em>x</em> = 1, <em>y</em> = 0 → d<em>y</em>/d<em>x</em> = (1 - 3)/(-3) = 2/3
<em>x</em> = 2, <em>y</em> = 0 → d<em>y</em>/d<em>x</em> = (2 - 3)/(-3) = 1/3
The slopes are not the same - they have to be if these tangents are supposed to be parallel - so (III) is not true.
This makes A. (I) only the correct choice.