Why are the the metabolic costs of an activity during the middle of the day higher than the metabolic costs of activity in the morning?
The metabolic costs of activity (not the actual metabolic rate), are at their highest during the middle of the day. It's because the cost to move around would be too high during the middle of the day that the lizard is inactive.
We know that the metabolic cost of activity midday is higher because the passage tells us that the cost is higher in the evening than in the morning, as a result of the lizard's body temperature being hotter in the evening than in the morning. Since temperatures are at their hottest midday, we can infer that body temperature is also at its hottest at this time and the metabolic cost of activity is at its greatest.
To explain further, we can find the answer by looking at the following two sections from the second paragraph:
The desert lizard is..."inactive during midday when external temperatures are extreme..,"
"...metabolic rates and water loss are great and sprint speed is lower in the evening when body temperatures are high. Thus, the highest metabolic and performance costs of activity occur in the evening when lizards have high body temperatures."
So, based on the above, we can infer that high body temperatures --> high metabolic rates --> high metabolic cost of activity.
Since we know body temperatures are higher during the middle of the day than in the morning (much higher actually), the metabolic cost of activity must also be higher than in the morning.
Remember, you're asked to make an inference, which means make the best assumption based on the knowledge given. Since you have a limited selection of assumptions (the answer choices), you have to ask yourself which one makes the most sense based on the passage. While the findings may be restricted to one specific animal, they point us in the direction of one assumption, and definitely don’t encourage us to make opposing assumptions.