The cyclic factors that can combine to create extreme seasons are called Milankovitch cycles, and involve several factors:
• Axial tilt or obliquity (how inclined or tilted is the Earth's rotational axis)
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• Axial precession (direction of the Eart's rotation)
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• Apsidal precession (the orientation of the Earth's orbit)
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• Orbital eccentricity (how elongated is the orbit)
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• Orbital inclination
We see that the sentence mentions them, let's use the options provided (perihelion, more, an increased) to complete the sentence.
For example, more extreme seasons could be caused by an increased tilt on Earth's rotational axis. Similarly, summer occurring at perihelion (closest to the sun) along a more eccentric orbit path would make for an extreme summer.