It's C because the argument uses the term vitamin deficiency in two different ways. First it says many pregnant women suffer from vitamin deficiency. So that means vitamin deficiency *for them* -- that is, the pregnant women aren't getting as many vitamins as pregnant women need.

Then it says this isn't due to vitamin deficiency in their diets. Here what the argument is trying to say is that the problem isn't caused by what would be a deficiency by the standards of the normal population -- the pregnant women *are* taking in the amount of vitamins that would normally be enough for non-pregnant women, but it's not enough for them, because pregnant women need more vitamins.

The idea that the argument is attempting to express seems clear enough, but it's expressing it in a muddled way. The first use of the phrase 'vitamin deficiency' obviously defines deficiency relative to what pregnant women need. The second use of the phrase defines deficiency relative to what adults normally need, outside of pregnancy. It makes the text confusing. C sums up the problem.