Which of the following is NOT an allotrope of carbon?

Methane

The correct answer is option 2. Methane.

Among the options provided, methane (option 2) is not an allotrope of carbon. Methane is a compound composed of one carbon atom and four hydrogen atoms \((CH_4)\), whereas graphite, diamond, and C-60 Buckminster fullerene are all allotropes of carbon.

So, the correct answer is 2. Methane.