Step 1: Recall the requirement for a photographic emulsion.
A photographic plate needs a substance that is sensitive to light, that is, one that changes chemically when light falls on it.
Step 2: Identify the light sensitive class.
Silver halides are the classic light sensitive compounds used for this purpose because light decomposes them to metallic silver.
Step 3: Pick the most suitable halide.
Among the silver halides, silver bromide $AgBr$ has the best balance of sensitivity, so it is the one coated onto photographic plates.
Step 4: Explain the role of gelatin.
The $AgBr$ is dispersed as fine colloidal particles in gelatin, which acts as a protective colloid and spreads the grains evenly over the plate.
Step 5: Write the light reaction.
On exposure, \[ AgBr \xrightarrow{light} Ag + \tfrac{1}{2}Br_2 \] and the tiny grains of silver form the latent image.
Step 6: Reject the other salts.
$CuBr$, $ZnBr_2$ and $FeBr_2$ are not photosensitive enough, so they are unsuitable. The answer is $AgBr$.
\[ \boxed{AgBr} \]