The word "substantial" most nearly means "sizable".
Something that is substantial is large or important in size, amount, or extent. In the case of the new job, the responsibility is sizable.
The other options do not have the same meaning as substantial.
- "Expensive" means costing a lot of money.
- "Lonely" means feeling sad or isolated because you are alone.
- "Small" means not large in size or amount.