18. The most convenient way of separating sawdust from water is
filtration
Explanation for correct option-
(C) Saw dust does not dissolve in water and hence, it can be separated from water using the filtration process.
Explanation for incorrect option-
(A) Distillation process is used to separate liquid mixture using selective boiling point followed by condensation. Sawdust does not dissolve in the water therefore, this process can not be used.
(B) This method isn't appropriate for the solids that aren't soluble in water.
(D) Sawdust cannot be separated from water using sedimentation or decantation since it is not heavier than water. Only if the insoluble material is heavier than the liquid may sedimentation and decantation be employed to separate it from the liquid.
Hence, option (C) is the correct answer.