what are the disc shaped membranous structures inside of the chloroplast where the light-dependent reactions take place

These are the thylakoids

Explanation:

The thylakoids which contains the chlorophyll is  the primary site of photosynthesis. It  is covered by layers of cell called the thylakoid membrane.These are the actual site for light dependent reaction.This membrane contains both photosytem I and ii, ATPase enzymes, and Cytochrome b6f complex.While the photosystem i and ii transfer elecrons during photosynthesis, the ATPase enzyme control the catalysis the phosphorylation of ADP to ATP needed for the light independent stage, the Cytochrome  proteins  acts as the ETC. involved in the transfer of electron during light dependent stage.

When thylakoids are packed together in disc shape they are refer to as Grana,(singular -granum).Each  grana are linked together by intergrana.Thus two or more grana are linked by intragrana to form a single photosynthetic functioning unit.The entire structure is bath continuously by the fluid medium-Stroma.

Their stacked and  disc shape is to increase the surface area available to accommodate enough  ETC, needed to transport electron for chemiosmosis needed for photosynthesis.

the answer will be a) proteins i believe