Hey there! As any English lover would do, let's get this question solved!
<span>Choice A: The illustrator drew his characters in pencil, then layered in more detail; ultimately he painted each comic strip in watercolor.
</span><span>Choice B: The illustrator drew his characters in pencil, then layered in more detail ultimately; he painted each comic strip in watercolor.
Choice C: </span><span>The illustrator drew his characters in pencil, then layered; in more detail ultimately he painted each comic strip in watercolor.
Once read over, We can see that answer choices B and C are incorrect.
That leaves us with Answer Choice A: </span>The illustrator drew his characters in pencil, then layered in more detail; ultimately he painted each comic strip in watercolor.
In other words the answer is Choice A: The illustrator drew his characters in pencil, then layered in more detail; ultimately he painted each comic strip in watercolor.