A compound containing a single primary nutrient is called ______ fertilizer

Correct option is C

Straight fertilizers supply only one primary nutrient (N or P or K).
Examples include urea (N), single superphosphate (P), and muriate of potash (K).
They are simple in composition and nutrient content is clearly defined.
Compound and complex fertilizers contain more than one nutrient.
Mixed fertilizers are physical mixtures of straight fertilizers.
Hence the correct answer is straight fertilizer.