Answer: C. COB.
Answer analysis: When discussing the types of commonly used outdoor LED lights, we need to consider the special requirements of outdoor environments for lamps, such as high brightness, good heat dissipation, weather resistance, and high protection levels. Now, let's analyze each option one by one:
A. SMD1010: SMD (Surface Mounted Device) is a surface mount device, which is a miniaturized package. SMD1010 refers to a package with a size of 1.0mm x 1.0mm for the LED chip. LEDs of this size are usually used for high-density, small-size display applications, such as mobile phone screens, small display screens, etc., rather than large outdoor lighting.
B. SMD1921: Also a type of SMD package, but with a relatively large size (1.9mm x 2.1mm). Although its brightness may be higher than SMD1010, it is still more suitable for high-density, refined display needs rather than large-scale outdoor lighting.
C. COB: COB (Chip On Board) is a chip-on-board package that directly packages multiple LED chips on a metal substrate, and achieves high-brightness and high-uniformity lighting effects through unified power supply and management. COB packaging has the advantages of excellent heat dissipation, high brightness, easy installation and maintenance, and is very suitable for large-scale outdoor lighting applications such as street lights and billboard lighting.
D. SMD1515: Similar to SMD1010 and SMD1921, this is also a miniaturized LED package, but its size (1.5mm x 1.5mm) is still more suitable for small, high-density display applications rather than outdoor lighting.
In summary, considering the special requirements of outdoor lighting for brightness, heat dissipation, weather resistance and protection level, COB packaged LED lights are the most suitable. Therefore, the correct answer is C. COB.