Here are the best-fitting words for each gap:

1. C. main
2. C. energy
3. A. able
4. A. lose
5. B. into
6. A. having

So the complete passage would read as follows:

Breakfast is the (1) main important meal of the day. It provides the nutrients your body needs for good health, the calories it needs for (2) energy and helps to maintain your blood sugar level. Studies have shown that people who don’t have breakfast have a low blood sugar level and are often slow, tired, and (3) able to concentrate. Surprisingly, breakfast actually plays a part in weight control. It’s easier to (4) lose weight if you eat in the morning rather than later in the day. Dividing the day's calories (5) into three meals helps take-off weight more efficiently than (6) having breakfast and having two larger meals a day does.