If 15% of a is equal to 25% of b, what is the ratio a:b?

The problem states that 15% of $a$ is equal to 25% of $b$.
We can write percentages as fractions or decimals:

$$\frac{15}{100}a = \frac{25}{100}b$$

Multiply both sides by 100 to remove the denominators:

$$15a = 25b$$

To find the ratio $a:b$, we need to express it as $\frac{a}{b}$. Divide both sides of the equation by $b$:

$$15 \frac{a}{b} = 25$$

Now, divide both sides by 15:

$$\frac{a}{b} = \frac{25}{15}$$

Both 25 and 15 are divisible by 5.

$$\frac{a}{b} = \frac{25 \div 5}{15 \div 5} = \frac{5}{3}$$

So, the ratio $a:b$ is $5:3$.

Comparing this with the given options:
a. 3:5
b. 5:3
c. 4:5
d. 5:4

The correct option is b.

The final answer is $\boxed{\text{b. 5:3}}$.