Boiling point elevation is a colligative property and depends on the number of particles (ions/molecules) in solution. CaCl$_2$ dissociates into 3 ions (Ca$^{2+}$ + 2Cl$^-$), contributing more particles than KCl (2 ions), glucose and urea (both non-electrolytes, 1 particle). Therefore, CaCl$_2$ solution has the highest boiling point.