Answer: Option (a) is the correct answer.
Step-by-step explanation:
Filtration is a technique or process in which solid that is suspended in a liquid is separated from the liquid.
For example, a muddy water can be filtered as particles of mud can be separated from the water.
Particles of salt being ionic in nature completely dissolve in water and it forms a homogeneous solution. Therefore, a mixture of salt water cannot be separated.
Thus, we can conclude that out of the given options salty water is the mixture which cannot be separated by filtration.