Some of the rootlets are specially adapted as negatively geotropic and are known as

The correct answer is Pneumatophores.

Explanation:

• Pneumatophores are specialized aerial roots that are adapted to grow upwards against gravity, hence they are negatively geotropic.
• These roots are typically found in mangrove plants, such as Rhizophora and Avicennia, which grow in swampy or waterlogged areas with poor oxygen availability in the soil.
• Pneumatophores help plants in gaseous exchange by allowing oxygen to diffuse directly into the plant's root system through specialized pores called lenticels.
• Pneumatophores roots grow vertically upwards and are adapted to absorb oxygen from the atmosphere. The presence of lenticels on these roots facilitates efficient gas exchange.

Other Options:

• Prop roots: These are adventitious roots that grow from the stem and provide additional support to the plant. They are seen in plants like the banyan tree (Ficus benghalensis).
• Stilt roots: Stilt roots are similar to prop roots but arise obliquely from the lower nodes of the stem, providing mechanical support. They are commonly observed in plants like sugarcane and maize. They are not negatively geotropic and do not function in oxygen absorption.
• Buttress roots: These roots are large, wide roots that extend from the base of the trunk to provide stability to tall trees, especially in tropical rainforests. They do not serve the purpose of gaseous exchange and are not negatively geotropic.