Given information:
- A number is decreased by 10% and becomes 63.
- We need to find the percentage by which the number should be increased to make it 84.

Let's solve the problem step by step:

Step 1: Find the original number
Let's assume the original number is x.

Given that, when x is decreased by 10%, it becomes 63.
So, the equation for this can be written as:
x - (10/100)x = 63
Simplifying this equation, we get:
(90/100)x = 63
Multiplying both sides of the equation by (100/90), we get:
x = (63 * 100) / 90
x = 70

So, the original number is 70.

Step 2: Find the percentage increase
Now, we need to find the percentage by which the number should be increased to make it 84.

Let's assume the required percentage increase is y%.

Given that, when the original number 70 is increased by y%, it becomes 84.
So, the equation for this can be written as:
70 + (y/100)*70 = 84
Simplifying this equation, we get:
(170/100)*70 = 84
Multiplying both sides of the equation by (100/70), we get:
y = (84 * 100) / (170/100)*70
y = (8400) / 119
y ≈ 70.588

So, the percentage increase required to make the number 84 is approximately 70.588%.

Step 3: Find the closest option
We need to choose the closest option from the given choices: a) 12%, b) 21%, c) 10%, d) 20%.

The closest option to 70.588% is 20%.

Therefore, the correct answer is option 'D' - 20%.

Conclusion:
To make a number 84 from 63, it needs to be increased by approximately 70.588%. The closest option to this percentage increase is 20%, which is the correct answer.