Final answer:
The complete subject of the sentence is 'Sandra's principal reason for choosing the job', which includes both the main noun and the modifying phrase providing context.
Step-by-step explanation:
The complete subject in the sentence 'Sandra's principal reason for choosing the job was that it would be full-time and would offer benefits' is 'Sandra's principal reason for choosing the job'.
A subject is the part of a sentence that tells us what or whom the sentence is about, and the complete subject includes all the words that describe the main noun. In this case, the subject is 'Sandra's principal reason' and the modifying phrase 'for choosing the job' is part of the complete subject, providing additional context about that reason.
The complete subject in this sentence is Sandra's principal reason for choosing the job. The subject of a sentence typically indicates who or what the sentence is about. In this case, Sandra's principal reason for choosing the job is the focus of the sentence.