Complex carbohydrates are a source of
nucleic acids
fats
vitamins
glucose

The correct answer for the fill in the blank is Glucose.

Complex carbohydrates are the macronutrients consisting of three or more sugars. They are also called as polysaccharides and oligosaccharides as they are made up of many monosaccharide units.

They are the primary source of energy for human beings and animals. Example- Starch, which is primarily made up of long chains of glucose or fructose. It is abundantly found in food like rice, wheat, and potato.

Glucose ( simple sugar) is a monosaccharide. Therefore, many glucose units combine and form polysaccharide or complex carbohydrates.  

Thus, the complex carbohydrates are a source of glucose.