Which Earth's axis property causes solstices and seasons?

Earth’s axial tilt causes solstices and seasons.

Earth’s rotational axis is tilted by about 23.4° relative to the plane of its orbit around the Sun. As Earth travels around its orbit, this tilt makes each hemisphere alternately lean toward or away from the Sun, creating the annual cycle of seasons and the two solstices.

Axial tilt changes both the angle of incoming sunlight and the length of daylight. When a hemisphere tilts toward the Sun, sunlight arrives more directly and days are longer; when it tilts away, sunlight is spread over a lower angle and days are shorter. A solstice occurs when this effect reaches its yearly extreme.

The seasons are not caused primarily by Earth being closer to or farther from the Sun, nor by orbital speed. Earth’s axis also slowly changes orientation through precession over roughly 26,000 years, while the tilt itself varies over much longer timescales between about 22.1° and 24.5°.