Answer: C: Environmental protection, long life and energy saving.
Answer analysis: This question mainly examines the advantages of LED lamp beads.
Option A, LED lamp beads do have high brightness, but this is not its most significant or unique advantage, because many other types of light sources can also achieve comparable brightness. Therefore, although option A is correct, it is not the best answer.
Option B, the relatively small size of LED lamp beads is one of its advantages, but in practical applications, small size is not always the only or decisive factor for "convenient use". In addition, this advantage is relatively minor compared to other advantages of LED lamp beads. Therefore, although option B describes a feature of LED lamp beads, it is not its most significant advantage.
Option C, the environmental protection, long life and energy saving of LED lamp beads are its most praised advantages. LED lamp beads produce almost no harmful substances during production and use, and are environmentally friendly; their service life is much longer than traditional light sources, reducing the frequency and cost of replacement; at the same time, LED lamp beads have a high energy efficiency ratio and can convert more electrical energy into light energy, thereby achieving energy saving. These advantages have made LED lamp beads widely used in the field of modern lighting. Therefore, option C is the correct answer.
Option D, although LED lamp beads may be cheaper and energy-saving in some cases, these advantages are not unique or the most significant. Especially in high-end LED lighting products, the price is not necessarily low. Therefore, option D, although partially correct, is not the best answer.
In summary, the correct answer is C: The advantages of LED lamp beads include environmental protection, long life and energy saving.