Which behavior in a client with schizophrenia demonstrates the client's cognitive inability to appropriately process data from external stimuli?

The answer would be d elusional beliefs

Schizophrenia is caused by high amount of dopamine inside the brain. It can cause the patient to percept something when there is no external stimulus taken (it different with the illusion where the external stimulus exists but perceived wrongly).

They also couldn't process the external stimuli, resulting in delusional beliefs.