Final answer:
The helping verb used to describe the mother's habitual Sunday activity of preparing food is 'will,' indicating a routine future action.
Step-by-step explanation:
The helping verb used to express what the mother usually does on Sunday is 'will'. A helping verb, also known as an auxiliary verb, assists the main verb to form a verb phrase.
In the sentence 'Mother will prepare some food to take with us', the helping verb 'will' indicates that the action of preparing food is a future event that occurs habitually every Sunday. This use of 'will' adds the sense that this is a regular activity by indicating a routine future action.
The helping verb is essential because it helps convey the tense or mood of the main verb, which in this case, is 'prepare'.