The correct option is C palm trees
There are two numbers in Noun-Number: Singular and Plural. When we speak about one person / thing, we use the noun in singular form. When we speak about more than one person / thing, we use the noun in the plural form.
In the given sentence, the subject is 'Raffia palms' which is plural. So, we need a plural noun. This rules out option A, which is singular. Options B, C and D are all plural. However, option B is incorrect because the word 'species' is already mentioned later in the sentence. So, if used, there would be repetition errors. Option D is incorrect because Raffia palm is a single type of tree, not types of trees. This leaves us with the answer as option C.