The correct choice is '手伝って' (te-form of '手伝う' - to help). The grammar pattern '〜てくれる' indicates that someone else does a favor for the speaker. Here, the senior coworker ('先輩') did the favor of helping the speaker ('私'). Sentence context: 'The senior staff at the shop always helped me when I was in trouble.' The other options are: '手伝わせて' (causative: making me help), '手伝われて' (passive), and '手伝わなくて' (negative), which are grammatically and contextually incorrect.