1. **D. most**
Explanation: The word "most" is used to indicate the highest degree or the greatest importance of something. In this context, "Breakfast is the most important meal of the day" means it is considered the meal of utmost importance.

2. **C. energy**
Explanation: The word "energy" is more commonly associated with the calories that food provides, which are necessary for performing daily activities and bodily functions. "Strength" and "power" can be related but don't fit as naturally in the context of calories from food.

3. **C. unable**
Explanation: The correct word here should contrast with being able to concentrate; hence "unable" fits perfectly: "are often slow, tired, hungry, and unable to concentrate."

4. **A. lose**
Explanation: In the context of weight control, the phrase "lose weight" is standard. Eating breakfast helps regulate metabolism, which can aid in weight loss.

5. **B. into**
Explanation: The phrase "dividing the day’s calories into three meals" makes it clear that the calories are to be spread throughout the day across three meals.

6. **A. skipping**
Explanation: The word "skipping" is appropriate as it refers to not having a meal. Hence, "skipping breakfast and having two larger meals a day" contrasts with eating three meals a day.