Because water has a relatively low viscosity, it will

I don't know

Allow substances to diffuse through it

First, you need to know what's viscosity. The more viscous a liquid, the more it'll flow easily. That will also go into accordance with what dumdum46 said. But the question is not specified, thus many relevant answer can be used. If it is in a certain chapter of your book, would you perhaps consider that. It'll guide you towards the required answer.

Unknown008 I think you have a typo/ have had a funny five minutes!

Viscousity refers to the thickness of a liquid therefore a more viscous liquid will flow less easily Viscosity (http://hypertextbook.com/physics/matter/viscosity/)

Oops, you're right templelane. I didn't realise my error:o