Step 1: Infection with the protozoan Toxoplasma gondii, especially when acquired transplacentally, prominently affects the eye and brain of the child. A well known triad combines retinal-choroidal inflammation, hydrocephalus and cerebral calcification.
Step 2: Within that picture, the signature eye finding is chorioretinitis, an inflammatory focus involving both the choroid and the overlying retina, frequently bilateral and capable of scarring the macula and reducing vision. This identifies option a as the answer.
Step 3: The remaining options describe inflammation of other ocular structures that are not characteristic of this parasite: the conjunctiva (conjunctivitis), the cornea (keratitis), and the optic nerve head (papillitis). They do not represent the typical toxoplasma lesion.
Step 4: So the expected ocular manifestation in a child is chorioretinitis.
\[\boxed{\text{Chorioretinitis}}\]