House is to room as tree is to:Forest, plant, leaf ?

Leaf.

Explanation:

A house has rooms.  A tree has leaves.  When completing these analogy exercises try to relate the words in a sentence, then test the options.  For example, if you first come up with the sentence "a house has rooms," use the same structure to test out the options for "tree."  There are "a tree has forests" which does not work, "a tree has plants" which does not work, so it must be "a tree has leaves."