At a critical point, a level curve is a single point or there are two or more level curves passing through the point.

Yes, that's true. I'm not sure why you consider that "ambiguous".This is even more ambiguous if you consider the lagrange multiplier method ..where both the the objective function and constraint function share the same tangent line/ tangent plane and both have the gradient vector perpendicular to their respective function level curves at the crtical points .... thats what a video lecture from MIT says....AM wrong in understanding may be...but would love to get clarified.... So pls reply

Consider the problem of finding a point on the graph of g(x,y)= constant that maximizes f(x,y). The gradient of the function, , points in the direction of greatest increase. If there were no constraint you would simply move in the direction of the gradient to get increasing values of f and eventually get to the maximum point.

But if you are constrained to stay on g(x,y)= constant, what you might do is look at the "projection" of the gradient vector onto the tangent line to the curve and move left or right depending on which way the projection pointed. That would take you at least a little closer-untilyou reached a point at which the gradient wasperpendicularto the curve. Then you could not get closer by moving either "right" or "left". And, of course, since is perpendicular to g(x,y)= constant, the two gradients must be parallel- must be a multiple of