Question

what do astronomers believe caused the sideways axial rotation of uranus?
the weight of the ice on its poles caused it to tilt.
it is a product of retrograde rotation.
it likely collided with another large body in space long ago.
any planet more than 15 au from the sun \adapts\ with a sideways axial rotation.

Explanation:

Astronomers widely accept that Uranus's sideways axial rotation (its axis is tilted ~98 degrees relative to its orbital plane) was caused by a massive collision with another large celestial body early in the solar system's history. This impact would have knocked Uranus off its original rotational axis. Other options are incorrect: ice weight does not cause such extreme tilting, nebulae rotation produces typical axial tilts (not sideways), and distance from the Sun (Uranus is ~19 AU but Neptune at ~30 AU does not have sideways rotation) does not determine this trait.

Answer:

It likely collided with another large body in space long ago