What will be the next letter in the pattern?
$$\text{E : H :: N : ?}$$

Let's find the alphabetical positions of the given letters:
* $\text{E}=5$
* $\text{H}=8$

The relationship between the first pair is:

$$\text{E}\stackrel{+3}{\to }\text{H}(5+3=8)$$

Applying the same pattern to $\text{N}$ ($\text{N}=14$):

$$\text{N}\stackrel{+3}{\to }14+3=17$$

The letter at position $17$ in the English alphabet is $\text{Q}$.