Intensity of central bright fringe due to interference of two identical coherent monochromatic sources is I. If one of the source is switched off, then intensity of central bright fringe becomes     

$\frac{I}{4}$

Let Intensity due to any one source is $I_1$.

Intensity of the bright fringe is $I = 4I_1$

When one of the source is switched off then intensity is $I_1 = \frac{I}{4}$