Can someone please answer this with explanation..!?

Dear Student

The Growth of a plant towards the light source is known as phototropism. It occurs in plants due to the presence of hormone auxin which is highly light sensitive. As sunlight falls on one side of the shoot, auxin gets stimulated and generally it diffuses to the shaded portion of the plant. It starts elongating the cells of the shaded portion. Thus the shaded portion grows and elongates more than the illuminated part. As a result the curvature of the plant stem tip bends towards the light.  
In case where gelatin is placed, the stem can easily bend towards the source of light.
However, the mica flake being impermeable, rigid and opaque at the same time prevents the stem from bending towards the light source since the mica doesn't allow the accumulation of auxin that would have helped in bending the stem.

Regards