The most likely diagnosis is (d): arachnoid cyst

The MRIs reveal a homogenous extra-axial (outside cerebral tissue) discrete mass lesion in the right lateral parietal location (which, on the coronal image [Fig. 2], is shown to arise from the temporal location of the middle cranial fossa). The mass lesion is hyperintense (bright) on the T2-weighted se-quences and equal in signal to the cerebrospinal fluid (CSF) within the lateral ventricles, in keeping with a cystic, rather than solid, lesion. There is secondary mass effect with displacement of the right cerebral hemisphere to the left, with sub-falcine shift, but the lateral ventricles remain fairly symmet-ric, suggesting a long-standing lesion (acute development of a mass of this size, with the same mass effect, would have resulted in effacement of the right lateral ventricle). No significant hydrocephalus is present. No evidence of hemorrhage or intra-axial edema.

Arachnoid cysts are benign, extra-axial, congenital collections of CSF covered by arachnoid cells of the CNS lining. Arachnoid cysts are seen in 4% of the population. Only 20% of these have symptoms, usually from secondary hydrocephalus. They account for about 1% of all atraumatic intrac-ranial mass lesions, and are discovered incidentally. Depending on size and location, they may cause neurological deficit through expansion that can compress normal neural tissue and obstruct CSF flow. They usually arise in areas that are rich in arachnoid, and 50% are located in the middle cranial fossa and region of the Sylvian fissure. Seizures and headache are said to be the most common symptoms of middle cranial fossa cysts.

The definitive treatment for arachnoid cysts is surgery and the absolute indications for surgery are the presence of progressive hydrocephalus or intracranial hypertension. The best operative inter-vention for children with arachnoid cysts remains the subject of controversy. Craniotomy with cyst fenestration into the basal cisterns (for middle cranial fossa cysts) has been shown to be associated with a low incidence of morbidity and mortality and may leave the child without a shunt device. The vast majority of these young patients will do very well with little or no residual effects.

The lesion in the images shows simple cystic characteristics and as such this would rule out astrocytoma and meningioma which are both typically predominantly solid lesions (and no contrast was given, so we can't assess whether the lesion enhances or not). The signal characteristics are not in keeping with acute subdural hemorrhage, and the lack of trauma history would make this extremely unlikely as well.

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