Concept:
The present simple tense describes habitual or general actions, while the past simple tense describes actions completed in the past.
Explanation:
The verb \textit{boils} is the present simple form of the verb \textit{boil}. To convert it into past simple tense, we use the past form \textit{boiled}. Therefore, the correct transformation is She boiled water for tea. The other options either use incorrect grammar or different tenses.