which of the following collection is not a set ?

Options

A. [x:x is a real number and `x^(2)-1=0`)
B. { x: x is a real number and `x^(2)+1=0`}
C. { x:x is a real number and `x^(2)-9=0`}
D. {x:x is a real number and `x^(2)=x+2`}

Explanation

we observe that there is no real number x such that `x^(2)+1=0`
`therefore ` {x:x is real number and `x^(2)+1=0}=phi`