Step 1: Understanding the Concept:
This question tests the knowledge of phrasal verbs. The correct phrasal verb must fit grammatically and contextually with the preposition "upon".
Step 2: Detailed Explanation:
The sentence expresses advice about how one should treat the poor. The structure is `look ... upon`.
- (A) look up: This phrasal verb means to search for information or to admire someone. It is not used with 'upon' in this context.
- (B) look down upon: This is a three-word phrasal verb meaning to regard someone with a feeling of superiority or contempt. The sentence "They should not look down upon the poor" makes perfect sense.
- (C) look after: This means to take care of someone. It is not used with 'upon'.
- (D) look on: This means to watch something without getting involved. It is not typically followed by 'upon' in this sense.
Step 3: Final Answer:
The correct phrasal verb that fits the sentence is 'look down upon'. Thus, 'look down' is the correct phrase to complete the expression. Therefore, option (B) is correct.