Explanation:

- To solve this, let's understand the concept behind the problem. When dealing with linear combinations of stamps or denominations, the biggest challenge is finding the smallest amount that cannot be purchased.

- Denominations available are Rs 7, Rs 8, and Rs 10. We need to figure out which option can't be expressed using these combinations.

- Option 1: 29 can be formed by 7+7+7+8.

- Option 2: 23 can be formed by 7+8+8.

- Option 3: 20 can be formed by 10+10.

- Option 4: 19 cannot be formed using any combination of Rs 7, 8, and 10.

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