Lichen is an association between two organisms – algae and fungus. Lichens are unique symbiotic organisms formed by a mutualistic relationship between an alga and a fungus. The alga provides food through photosynthesis, while the fungus provides a protected environment and absorbs water and nutrients.

Option 2, algae and tree, is incorrect as lichen does not form a symbiotic association with trees. Lichens can grow on tree bark, but they are not directly associated with the tree itself.

Option 3, fungus and tree, is also incorrect for the same reason mentioned above. Lichens can grow on trees, but they are not directly associated with trees.

Option 4, bacteria and legume plant, is unrelated to the association between algae and fungus in lichen.

Therefore, the correct answer is option 1 – algae and fungus.