A primary source is an original document or firsthand account created at the time of an event or experienced firsthand by an individual. Among the examples you provided, a soldier’s letters home from war would be considered a primary source. It provides direct evidence and personal insights into the experiences of the soldier during that particular time.
The other options, such as an internet article, a movie, or a teacher's lecture, are considered secondary sources as they interpret, analyze, or summarize events rather than present firsthand accounts.