The . . . . . . . . in a pattern represents one character only.

The correct answer is $\boxed{\text{?}}$.

The question is asking which of the following symbols represents one character only in a pattern.

• $\text{?}$ represents any single character.
• $\text{*}$ represents any number of characters.
• $\text{#}$ represents any non-alphabetical character.
• $\text{**}$ represents any two characters.

Therefore, the correct answer is $\text{?}$.