Final answer:
The sentence in question is written in the passive voice, as the subject 'she' receives the action of the warning, and the sentence uses the past participle 'warned' with a form of 'to be'. In comparison, the active voice would directly state who acted.
Step-by-step explanation:
The sentence 'She was warned that watching TV for extended periods of time could have negative effects on her health' is written in the passive voice. This can be determined by identifying that the subject 'she' is not performing the action but is being acted upon by being warned. Additionally, a form of the verb 'to be' (in this case, 'was') is used together with the past participle 'warned'.
There is no 'by [noun phrase]' explicitly stated in the sentence, but it could be added, which further confirms that the sentence is passive. For example, it could be rephrased to 'She was warned by a doctor...' without changing the meaning. Unlike active voice where the subject acts, such as 'The doctor warned her that watching TV...', the passive voice focuses on the action happening to the subject.
Most people now spend several thousand hours a year sitting still and staring at the screen. It is acknowledged that excessive screen time is linked to various health issues. However, in writing, using the active voice typically makes sentences clearer and more direct, while the passive voice can sometimes lead to vagueness or a lack of vitality in the text.