Final answer:
The suffix 'sion' should be added to 'expedi' to create the word 'expedition'. This suffix causes the 't' to be pronounced as [ch].
Step-by-step explanation:
The correct suffix to be added to the word expedi to form a complete word is A) sion. This will result in the word expedition, which means a journey undertaken by a group of people with a particular purpose, especially that of exploration, research, or war. The suffix '-sion' when added to the base 'expedi' causes a palatalization, changing the sound of 't' in 'expedi-t-ion' to be pronounced as [ch], which is common in English language patterns when 'ion' follows a base word ending with 't'.