Answer
2006 to 2007
Work Step by Step
2002: 29.0 miles per gallon
2003: 29.5 miles per gallon
2006: 30.25 miles per gallon
2007: 31.0 miles per gallon
2008: 31.25 miles per gallon
From 2002 to 2003,
$\frac{29.5-29.0}{2003-2002}=\frac{0.5}{1}=0.5$.
From 2006 to 2007,
$\frac{31.0-30.25}{2007-2006}=\frac{0.75}{1}=0.75$.
From 2007 to 2008,
$\frac{31.25-31.0}{2008-2007}=\frac{0.25}{1}=0.25$.
2006 to 2007 has the greatest slope.