If x^2+kx+1<0 for all real values of x, then k belongs to
d.None of these
The graph of y = x2 + kx + 1 is a parabola with its minimum point at its vertex and rises, on both sides, from there.
There is no way that this parabola will always be below zero; that is, will always be below the x-axis. So, the answer is the null set.
If the graph were: y = - x2 + kx + 1, which is a parabola with a maximum point and falling, it would be possible for the graph to be always below the x-axis.