For the first question, write the full word with an 's' on the end. For example, if it was domesticated animals and the letter is 'C -' then you should write 'Cats' not 'ats' or 'at' .
For the question about receptors, write the singular organ, not the plural. For example, if it was domesticated animals, and the sentence is "kittens are the offspring of the" then you should write 'cat not 'cats'.