Vegetables 2

I have not heard about cassava before. Actually, I do not like to eat veggies very much. But I am getting to use to like them for my health.
Cassava means ''manyok'' in Turkish, just like ''manyak'' which means ''maniac'' in English.

I don't like pumpkins and I've never eaten cassava, but I particularly like artichokes, that are suitable for several ways of cooking: stuffed, fries, omelette, with pasta, etc.

The vegetables I like the most are green beans, spinach, garlic, ginger, pumpkin and radish. The vegetables I like the least are leeks, courgettes, chard and broccoli.

Of these, I like boiled broccoli the most, I don't like leeks.

I like most of those vegetables. As I like garlic and pumpkin the most, I like chard the least. I am fond of baked pumpkin with syrup, walnuts and sesame butter on the top. Moreover, I have heard "cassava" for the first time.

I really enjoy boiled broccoli and cassava soup. I don't like the garlic, especially raw one.

Leeks and pumpkins are those vegetables ı like the most. I use oregano and pepper as spices to give flavor to food. I haven't tasted artichoke and chard.

In task 3, I can't make sure when to use plural when to use singular. e.g. 1 We eat the leaves of chard and spinach. (Why do the answer using singular nouns?) Why my answer (chards and spinaches) is wrong? 2 The part of radishes and leeks that we eat grows partly above ground and partly below. (Why do the answer using plural nouns?) 3 The part of garlic and ginger that we eat also grows below ground. (Why do the answer using singular nouns this time? I believe it is the same structure as the sentence 2) I am totally confused about these usages.