The correct answer is Will arrest.
In English syntax and grammar, this sentence structures a classic First Conditional framework. This specific grammatical construction is universally employed to depict an entirely realistic future consequence that relies directly upon a specific condition being met. The foundational rule for the first conditional requires the conditional clause (the \"if\" clause) to be articulated in the simple present tense (\"if he doesn\'t stop\"), while the main consequence clause must be constructed using the simple future tense (\"The policeman will arrest\").
The alternative tense selections violate these established syntax guidelines for predictive conditional sentences. The simple present tense arrests would incorrectly imply a universal truth, habitual routine, or timeless scientific fact rather than a specific future event. Furthermore, selecting the past tenses—the simple past arrested or the past perfect had arrested—creates a severe chronological mismatch with the present-tense condition. Therefore, the future tense will arrest is the only choice that maintains flawless grammatical harmony and logical temporal progression.