Fill in the blank with the most appropriate phrasal verb: "My younger brother _____ my grandfather." Choose the phrasal verb that makes the sentence meaningful and grammatically correct.

Introduction / Context:
This question tests your understanding of common English phrasal verbs, especially those built with the verb "look". Each particle or preposition (on, after, to, of) changes the meaning of "look" in a specific way. The sentence is about a younger brother and his relationship to his grandfather, so the correct phrasal verb must express the idea that he takes care of or tends to his grandfather.

Given Data / Assumptions:

- Sentence: "My younger brother _____ my grandfather." - Subject: "My younger brother". - Object: "my grandfather". - Options: "looks on", "looks after", "looks to", "looks of". - The natural context is of care, responsibility, or emotional relation.

Concept / Approach:
The phrasal verb "look after" means "to take care of" someone or something. It fits perfectly when describing a person who helps, protects, or looks out for another, especially an older family member. The other options have different meanings or are incorrect combinations: "look on" means to watch without participating, "look to" can mean to depend on or expect something from someone, and "look of" is not a standard phrasal verb in English.

Step-by-Step Solution:
Step 1: Think about the likely meaning: in many families, a younger person may help "take care of" an older grandparent. Step 2: Recall the meaning of "look after": to take care of someone, as in "She looks after her younger sister." Step 3: Insert "looks after" into the sentence: "My younger brother looks after my grandfather." This is grammatical and meaningful. Step 4: Test "looks on": "My younger brother looks on my grandfather" is incorrect. "Look on" usually appears as "look on at the game", meaning to watch something, not to care for a person. Step 5: Test "looks to": "My younger brother looks to my grandfather" suggests that he depends on or seeks advice from his grandfather. While this can be meaningful in some contexts, exam questions of this type usually target the well known phrase "looks after" for care and responsibility. Step 6: Test "looks of": "looks of" is not a standard phrasal verb and is clearly wrong. Therefore, "looks after" is the best and intended answer.

Verification / Alternative check:
Compare similar sentences: "She looks after her grandparents", "They look after the house when we are away". In each example, the sense of care and responsibility is clear. Replacing "look after" with "look on" or "look of" would immediately sound wrong. Even "look to" does not convey the idea of care; it refers more to reliance. This confirms that the expected meaning in the question is about looking after an elderly relative.

Why Other Options Are Wrong:
"Looks on" means to observe a situation as a spectator and does not take a direct object in the same way here. "Looks to" usually means to rely on or expect help or guidance from someone, which shifts the meaning away from physical care. "Looks of" is not a recognised phrasal verb in English and produces an ungrammatical phrase. Only "looks after" fits smoothly and gives a clear, natural sentence.

Common Pitfalls:
Because many phrasal verbs begin with "look", learners sometimes mix them up, relying only on the main verb instead of remembering each full expression. A good way to avoid confusion is to learn phrasal verbs in context with example sentences and to link each one to a specific meaning: "look after" for care, "look up to" for respect, "look into" for investigate, and so on. In exam questions, always check both grammar and meaning before choosing your answer.

Final Answer:
looks after correctly completes the sentence as "My younger brother looks after my grandfather."