The Long March Solid Boosters are small-sized boosters that are part of the Long March rocket. They are one of the boosters which stays on even when the main stage is turned off.
In real life ,the long march rocket uses liquid fueled kerolox boosters, as opposed to the game's solid fuel.
The long march boosters also have an extended version in real life that is used on the Long Matches 5,7, and (proposed) 9.
The boosters are the most similar to those used on the Long Marches 2E and 2F.