To obtain pure water from a mixture of sand, water, and methanoic acid, we need to consider the properties of each component:

- Sand is insoluble in water and can be removed by filtration.

- Methanoic acid (formic acid) is a volatile liquid that can be separated from water by distillation.

- Water is the desired product.

Considering these properties, the correct answer is: Option D. (Filtration followed by distillation)

Here's the step-by-step process:

1. Filtration: Remove the sand from the mixture by passing it through a filter paper.

2. Distillation: Distill the resulting liquid mixture to separate the methanoic acid from the water. Since water has a lower boiling point than methanoic acid, it will vapourize and condense first, allowing you to collect the pure water.