Step 1: Recall what a mushroom is.
A mushroom is the visible fruiting body produced above the ground by certain fungi. The main body of the organism is a network of thread like structures called hyphae, which stays hidden in the soil or wood, and the mushroom cap is only the reproductive part that releases spores.
Step 2: Place mushrooms in the right kingdom.
Fungi lack chlorophyll and cannot make their own food, so they live by absorbing nutrients from dead or living organic matter. Mushrooms show all these traits, so they are classified in the kingdom Fungi.
Step 3: Rule out the other options.
(A) Algae: Algae are simple, mostly aquatic organisms that contain chlorophyll and carry out photosynthesis to make their own food, unlike mushrooms which cannot photosynthesize. Incorrect.
(B) Ferns: Ferns are vascular plants with true roots, stems, and leaves, and they too are photosynthetic and reproduce by spores from fronds, which is a very different body plan from a mushroom. Incorrect.
(D) Lichens: A lichen is a partnership between a fungus and a photosynthetic partner such as algae or cyanobacteria living together; a plain mushroom is only a fungus by itself, without this partnership. Incorrect.
Step 4: Final answer.
Mushrooms are fungi, built from hyphae and lacking chlorophyll, reproducing through spores.
\[ \boxed{\text{Fungi}} \]