The description "a forest-like arrangement of columns supporting the roof" refers to a specific architectural feature found in ancient temple architecture, particularly Egyptian.
Let's examine the options:
(a) Pylon: In ancient Egyptian architecture, a pylon is a monumental gateway to a temple, typically consisting of two massive, tapering towers with a central doorway. It's an entrance structure, not a hall filled with columns.
(b) Hypostyle Hall: "Hypostyle" (from Greek \textit{hypostylos}) means "under columns." A hypostyle hall is a large interior space whose roof is supported by many closely spaced columns or pillars. This dense arrangement of columns can evoke the image of a forest. Famous examples include the Great Hypostyle Hall at the Karnak Temple Complex in Egypt.
(c) Obelisk: An obelisk is a tall, four-sided, narrow tapering monument which ends in a pyramid-like shape (pyramidion) at the top. They were often erected in pairs at the entrances of ancient Egyptian temples. It's a monolithic monumental pillar, not a hall.
(d) Mastaba: A mastaba is an ancient Egyptian tomb, rectangular in shape with sloping sides and a flat roof, built of mud-brick or stone. It was a precursor to pyramids. It's a tomb structure, not a hall characterized by a forest of columns.
The architectural feature characterized by a "forest-like arrangement of columns supporting the roof" is a Hypostyle Hall.
\[ \boxed{\text{Hypostyle Hall}} \]