Which of the following is cheaper fertiliser?

Urea (Option B) is typically considered a cheaper fertilizer compared to other options listed. It is a widely used nitrogenous fertilizer due to its relatively low cost of production. Single super phosphate (Option A), dicalcium phosphate (Option C), and ammonium sulfate (Option D) generally tend to be more expensive than urea. Hence, the correct answer is Option B: Urea.