They are the thin, frail women who don't need to exercise. They are as straight as a ruler with little or absolutely no curves at all. There is no difference between the waist, chest and hip size. They should eat a lot of proteins, a lot of calcium and complex carbohydrates. Such body types need not bother much about their weight.
Ideally, they should do simple stretches, sit-ups, crunches, squats or basic yoga to keep themselves fit.
Knowing what will work for your body shape is perhaps the first and most significant step to take before you adopt a fitness regime. After all, a workout well begun is almost half the battle won !