2% of a = b, then b% of 10 is the same as:

We are given that $$2\%$$ of $$a$$ equals $$b$$.
By the definition of percentage, this means

$$2\% \text{ of } a = \frac{2}{100}\,a = b$$

Simplifying,

$$b = \frac{a}{50} \quad -(1)$$

Now we must evaluate “$$b\%$$ of $$10$$”.
A percentage $$x\%$$ of a number $$N$$ is $$\frac{x}{100}\,N$$. Therefore,

$$b\% \text{ of } 10 = \frac{b}{100}\times 10 = \frac{10b}{100} = \frac{b}{10} \quad -(2)$$

Substitute the value of $$b$$ from $$(1)$$ into $$(2)$$:

$$\frac{b}{10} = \frac{1}{10}\left(\frac{a}{50}\right)=\frac{a}{500} \quad -(3)$$

Next, compare $$(3)$$ with the options. We observe that

$$20\% \text{ of } a = \frac{20}{100}\,a = \frac{a}{5}$$

If we now divide this by $$100$$, we get

$$\frac{1}{100}\left(\frac{a}{5}\right)=\frac{a}{500}$$

This matches the value in $$(3)$$ exactly. Hence “$$b\%$$ of $$10$$” equals “$$20\%$$ of $$a/100$$”.

Therefore, the correct choice is:
Option B which is: 20% of a/100