The polynomial ax2+bx+c is a linear polynomial if
a = 0
A linear polynomial must be of the form ax+b where a≠0.
The degree of the given polynomial ax2+bx+c is 2, when a≠0.
When a=0 the polynomial reduces to the form bx+c and will be a linear polynomial
But when a=0 and b=0 the polynomial will be of the form c which is a constant.
Thus, when a=0 and b≠0, ax2+bx+c will be a linear polynomial.