Direct means both quantities move together. Inverse means one rises while the other falls. ACT Science uses those words in the choices. You do not need a formula. Trend reading is in interpreting trends.
What the line is doing
If x goes up and y goes up, the relationship is direct. If x goes up and y goes down, it is inverse. A line that rises and then falls is neither for the whole graph. Limit yourself to the interval the question asked about before you pick “direct” or “inverse.”
“Increased only” and “decreased then increased” are the usual traps. They describe shape, not a vocabulary word. Peaks and lows are in minimums, maximums, and turning points.
Do not import math class
You rarely need y = kx or y = k/x. The figure already shows the direction. Data-representation passages, where these graphs live, are in data representation. Faster graph reading is in reading graphs faster.
The bottom line
Same direction is direct. Opposite direction is inverse. Say that only for the interval in the question.
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