Fill in the blank.
The clever trader worked ________ his customers.

Correct option is C

Correct option: (c) upon
Meaning of the phrasal verb (work upon):
• Work upon (someone) → to influence/manipulate/persuade; to act on someone’s mind/feelings (Hindi: प्रभाव डालना / बहकाना / मन बदलना)
• Why it fits: A clever trader can influence customers to buy goods.
Correct sentence: The clever trader worked upon his customers.
Grammatical Rule (English):
• “Work upon” is a fixed phrasal verb; correct particle is necessary for correct meaning.
व्याकरणिक नियम (Hindi):
• “Work upon” एक निश्चित phrasal verb है; सही अर्थ के लिए सही particle “upon” ही लगेगा।
Meanings of other options (in detail):
• (a) work into → to gradually introduce/include; to blend into (Hindi: धीरे-धीरे शामिल करना)
• (b) work up → develop/prepare gradually; excite/anger (Hindi: तैयार करना / उत्तेजित करना)
• (d) work out → solve/figure out; calculate; plan successfully (Hindi: हल निकालना / समझ लेना)
Hence, option (c) is the correct answer.