Select the most appropriate option to fill in the blank.

Though he is stout, he runs ____________.

fast

The correct option to fill in the blank is fast.

The word "fast" means to move or travel at a relatively high speed. It is the most appropriate word to use in this context, as it conveys the meaning of the stout person being able to run at a relatively high speed despite his size.

The other options are not as appropriate.

• Faster means at a relatively faster speed. This is not the right word to use, as it does not make sense to say that the stout person runs faster than everyone else.
• Fastest means at the fastest speed possible. This is not the right word to use, as it is not necessary to say that the stout person runs at the fastest speed possible.
• Fastly means in a fast manner. This is not the right word to use, as it is not a noun.