Explanation

(c) In Young’s double slit experiment, for maximum intensity (bright fringe)
$x = m\frac{D\lambda}{d}$
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Where m is path difference, D the distance between screen and coherent sources, d the distance between coherent and $\lambda$ the wavelength.
Putting $m = 0,$ we get the position of the central bright fringe (which is called zero order fringe). Hence, at point O the path difference between two wavelets is zero. Hence, at O there is always a bright fringe. This is called the central fringe.