Jonny's first step in solving the equation \( 8 + 3n = 20 \) is to subtract 8 from both sides in order to isolate the term with \( n \). The correct reasoning for this step is not fully captured by any of the provided options, but the closest explanation would be:
He is trying to isolate the variable on one side of the equation.
By subtracting 8 from both sides, he can simplify the equation to:
\[ 3n = 20 - 8 \] \[ 3n = 12 \]
This makes it easier for him to solve for \( n \) in subsequent steps.