Explanation:

- Notion refers to an idea, belief, or understanding about something. It often implies a concept that might not be based on reality.

- Option 1: Quotient is a result of division in mathematics. It has no direct antonymous relationship with notion.

- Option 2: Law is a system of rules. It's not an antonym of notion since both can coexist in ideas and beliefs.

- Option 3: Reality refers to the state of things as they actually exist. It is indeed an antonym of notion when notion represents something imagined or abstract.

- Option 4: Portion is a part or segment of something. It does not contrast with notion.

The antonym of "notion" in this context is Reality.