~のひとつ (one of)

The pattern [X]のひとつ, meaning 'one of (the) X'. Common in both formal and casual writing for asserting membership in a set: ~は~のひとつだ/である ('X is one of Y'). The fuller form ~の一つである — kanji-written counter plus formal copula である — appears in more formal/written contexts but is the same construction.