Answer:
$4.60 per kilogram
Explanation:
A slope is a ratio of one value to another. On a graph, it is the ratio of the vertical distance (rise) to the corresponding horizontal distance (run). It is always <something> per <something else>. Usually, we want to express a slope such that the <something else> is a single unit. The ratio may have to be reduced to get the value of the slope in those terms.
Here, the problem statement tells you the postage rate is some fixed value plus <$4.60> per <kilogram>. That is, the slope is $4.60 per kilogram.
_____
Additional comment
When a problem asks about "this situation," you are always faced with the task of determining exactly what aspect of the described situation is the aspect of interest.
Here, it appears that we are to consider the graph of postal charges versus kilograms of weight. It will be a straight line with an initial value (y-intercept) of $10.30. The slope of the line on the graph will be the given rate per unit weight of the item.