Opposite of earning Rs. 100 is spending Rs. 100.

Explanation:
Earning Rs. 100 means acquiring or receiving Rs. 100 as income or profit. On the other hand, spending Rs. 100 involves utilizing or giving away Rs. 100 to purchase goods or services. Let's break down the concept further:

Earning Rs. 100:
- When you earn Rs. 100, it implies that you have gained Rs. 100 as a result of your work, investment, or any other source of income.
- Earning money typically involves receiving payment for goods sold, services rendered, or work performed.

Spending Rs. 100:
- Conversely, spending Rs. 100 means you are using Rs. 100 to purchase something.
- Spending money is a common activity where individuals exchange their currency for goods, services, or experiences.

Opposite Relationship:
- Earning and spending are opposing actions when it comes to money management.
- When you earn money, you accumulate wealth or increase your financial resources.
- Conversely, spending money reduces your financial resources as you exchange it for goods and services.

Options Analysis:
a) Rs. 100: This option represents the same amount of money, so it cannot be the opposite of earning Rs. 100.
b) Profit of Rs. 100: Profit refers to the positive difference between revenue and expenses. It is not the opposite of earning Rs. 100.
c) Gain of Rs. 100: Gain is similar to earning and does not represent the opposite action.
d) Spending Rs. 100: This option aligns with the opposite relationship as explained above. When you spend money, you are utilizing it rather than earning it.

Therefore, the correct answer is option 'D' - Spending Rs. 100.