Answer
(C)
Work Step by Step
A line in slope-intercept form has the equation:
$y=mx+b$ ($m=slope$, $b=y-intercept$)
We convert the line equation to slope-intercept form:
$x+3y=3$
$3y=3-x$
$y=\frac{1}{3}(3-x)$
$y=\frac{1}{3}*3-\frac{1}{3}x$
$y=1-\frac{1}{3}x$
$y=-\frac{1}{3}x+1$
So the line has a $y$-intercept of 1 and a negative slope. Only (C) matches these properties.