In a single spool engine, the compressor and the turbine that drives it are mounted on one shaft that rotates at a single speed for the whole engine. In a multiple spool engine (twin spool or triple spool), the compressor is split into a Low Pressure (LP) section and one or two High Pressure (HP) sections, each mechanically independent and driven by its own turbine through a concentric shaft. Each spool can therefore rotate at the speed that is aerodynamically optimum for its stages: the LP compressor turns relatively slowly, while the HP compressor turns much faster. 

This decoupling has several important consequences. Only the HP spool needs to be spun up during engine start, so the air starter can be smaller and lighter than one designed to spin the entire rotating assembly of a single spool engine. The multi spool architecture also improves the surge margin, makes higher overall pressure ratios possible, and improves engine efficiency. 

A compressor stall cannot occur under any condition → INCORRECT. The multiple spool architecture improves the surge margin because each spool can adjust its speed independently to match the airflow, but it does not eliminate the risk of compressor stall. Rapid throttle inputs, inlet distortions or damaged compressor blades can still cause a stall.

A smaller air starter driving only a single spool can be used → CORRECT. In a multi spool engine, the air starter only needs to drive the HP spool up to the self sustaining speed. The LP spool is dragged along by the airflow as the HP spool spins up and combustion starts. This allows the use of a smaller, lighter air starter than would be required to accelerate the entire rotating assembly of a single spool engine.

If one spool seizes, the remaining spool(s) will continue to operate normally → INCORRECT. A seized spool would block the airflow through the engine and stop the entire engine, regardless of the mechanical independence of the shafts. The spools are aerodynamically dependent even if they are mechanically decoupled, so a single seized spool would still cause a shutdown.

Engine length can be reduced → INCORRECT. Multi spool engines are usually longer, or at least of similar length, compared to single spool equivalents. Adding a second (or third) spool adds concentric shafts and additional turbine stages, which does not reduce the overall length of the engine.