Given an entity as input, output another entity which is part of the input entity. These are entities of meronym. In linguistics, meronymy is a semantic relation between a meronym denoting a part and a holonym denoting a whole. In simpler terms, a meronym (i.e., output entity) is in a part-of relationship with its holonym (i.e., input entity).
Example: plant
Example solution: stem
Example explanation: The answer is correct. Because a stem is one of two main structural axes of a vascular plant, the other being the root. Therefore, stem (i.e., meronym) is the part of plant (i.e., holonym).

Problem: nitrogen fix tree

Solution: taproot