Option 1 (B) If you want a focal point of 2 meters (which will exceed your height requirement), you would have a parabola height of 1/32 meter. (Bad option because the skewer that holds the food is too high) Equation y = (x^2)/8
Option 2(A) If you want a focal point of 1/16 meter, you would have a parabola height of 1 meter (which, once again is on the edge of the height requirement but places your food well inside the parabola) Equation y= 4x^2
KEEP IN MIND - ALL of the above options take advantage of all the sun possible. There is no advantage in 'power' of sunlight from one to the next. They will all heat identically.
So-
What we find the equation -
y= (x^2)/(4f)
is the equation you need with the added restriction of
1 (>=) f (>=) 1/16 (Read: "One is greater than or equal to 'f' which is greater than or equal to '1/16')
Comment: I would think that while the above equation satisfies the math, the best option is somewhere closer to option1 (A), so that the food is more accessible.
Hope that helps!