単なる
たんなる · tannaru · JLPT N3 · na-adj
mere
単 (single, simple) + なる (classical 'that is') = that is simply
単 means single or simple, as in 単語 (word) and 簡単 (easy). なる is the old classical form of 'that is', so 単なる literally means 'that is simple only', which gives 'mere'. It always comes right before a noun: 単なる噂, a mere rumour.