Choose the set of numbers from the four alternatives that is similar to the given set.
Given set: $(21,51,15)$

In the given set $(21,51,15)$, all three numbers are multiples of $3$:
* $\frac{21}{3}=7$
* $\frac{51}{3}=17$
* $\frac{15}{3}=5$

Notice that all the resulting quotients $(7,17,5)$ are prime numbers.

Let us test the options for a similar pattern:
In option D, the set is $(21,91,35)$. All three numbers are multiples of $7$:

* $\frac{21}{7}=3$
* $\frac{91}{7}=13$
* $\frac{35}{7}=5$

The resulting quotients $(3,13,5)$ are also all prime numbers, following the exact same logic.