Although a book might be teaching a response to "What is that?" as "It is a...." It is perfectly acceptable to answer it with "That is a ..." For example: What is this? It is a ball. What is this? This is a hat. What is that? That is a desk. Regarding "these" and those" that is the plural form of "this" and that" . Now that the subject is plural the verb must be plural too so that "is" changes to "are". What is this? This is a ball. What are these? These are balls. What is that? It is a snake. What are those? Those are dogs.

Hello Irina, When asking : "What is this ?" or " What is that ?" The answer could be either : "It is ......" or "This is ......" In case of plural it is almost the same ,yet the verb will be altered: "What are these?" "What are those?" "These are ....." Here you can't answer by saying " It is ...." but you can say "They are ..."

What's this/that? is the simplified form of what is this/that? Purals, change the "is" to " are" What're these/those ?