The weather systems that may form precipitation in the figure are () a, 1B, 2C, 123D and 123④.

Test sites: air pressure system and weather front system and weather topics: analysis: mainly investigated air pressure, front system and weather, ① cold air mass actively moved to warm air mass to form a cold front; ② The warm front is formed when the warm air mass actively moves to the cold air mass; (3) The airflow rises in the vertical direction, and it is easy to condense clouds during the rising process, causing rainfall, which is a cyclone; ④ The airflow sinks vertically, and the temperature rises during the sinking process, which is an anticyclone. ① Cold front: The front edge of the cold air mass is inserted under the warm air mass, and the warm air mass is forced to rise, and the water vapor inside is easy to condense, resulting in rainfall; ② Warm front: When crossing the border, rainy weather often occurs; (3) Cyclone: The airflow rises vertically, and it is easy to condense clouds during the rising process, resulting in rainfall; ④ Anti-cyclone: the airflow sinks vertically, and the temperature of airflow rises during the sinking process, mostly in sunny days. So choose: C. Comments: This question is less difficult and belongs to the knowledge test. The key to solve this problem is to master the air pressure, frontal system and weather.