Hadith comprehension

What is “فَرِيضَةٌ” in “طَلَبُ الْعِلْمِ فَرِيضَةٌ”?

Travel, repayment, service, and sufficiency (Hadith 37–43)

• A conjunction
• The predicate of the nominal subject
• A past-tense verb
• A sound masculine plural