Rubber plantations are primarily found in equatorial regions. Equatorial regions are characterized by a tropical climate with high temperatures and abundant rainfall, providing optimal conditions for rubber tree cultivation. These regions are typically located near the equator, and examples include countries like Malaysia, Indonesia, Thailand, and Vietnam.

Temperate forests, option 1, are characterized by temperate climates and are typically found in regions with moderate temperature ranges and distinct seasons. Mountain regions, option 2, generally have colder climates and higher elevations, making them unsuitable for rubber plantations. Polar regions, option 3, are extremely cold and have permanently frozen ground, making them inhospitable environments for rubber tree growth.

Overall, option 4, equatorial regions, is the correct answer as these areas provide the ideal conditions for rubber plantations to thrive.