Translation:This pair is smaller than that pair.
If you mean "this pair is smaller than that one", I'm sure I've said it that way many times myself but I think it's slightly incorrect. Even though we can say "a pair" or even "one pair" of something; if expand this to "... than that one pair" it becomes rather odd (we're not counting the number of pairs but comparing the two pairs) So I think "this pair is smaller than that" would be the best way of saying this. But "pair" deffinatly needs to be there (see the discussion about "two" below)
The English grammar in this answer is totally wrong. When it is a pair of objects we usually refer to it as a plural. So the answer should be: "This pair 'are' smaller than that pair.
If you don't believe me here is the explanation:
"Nouns for articles of dress consisting of two parts are . . . treated as plural: [A] Where are my trousers? [B] They are in the bedroom where you put them. But such plural nouns can be 'turned into' ordinary count nouns by means of a pair of or pairs of: I need to buy a new pair of trousers. How many pairs of blue jeans do you have?" (Geoffrey Leech and Jan Svartvik, A Communicative Grammar of English, 3rd ed. Routledge, 2013) "Suppose we're talking about things like scissors, tongs, glasses, or trousers. Scissors, tongs, glasses, and trousers are odd words; even though we might be, and indeed usually are, talking about one pair of trousers at the message level, we treat trousers as a plural, and therefore have to mark the verb as plural. We always say 'the trousers are too long,' and should never say 'the trousers is too long.' Talking of trousers, pants behaves the same way; we say 'here are your pants,' not 'here is your pant.' Just to complicate things more, the plural of these words stays the same, so we can say: Here is your pair of pants. Here are your pants. Here are your two pairs of pants.I have just glued your pants together, and here are two pairs of pants. Such words are called pluralia tantum (singular plurale tantum), and yes, they are confusing." (Trevor A. Harley, Talking the Talk: Language, Psychology, and Science. Psychology Press, 2010)
Of course Duolingo who seem to think sometimes that we are here to learn English and not Chinese often make mistakes like this.
If you answer "this pair are smaller than that pair" you get marked wrong, but it is Duolingo that is wrong.
@George - whilst "trousers" are treated as plural, the "pair" is treated as singular. We usually wear ONE pair of trousers at a time. Not two pairs. You can of course assume that the given sentence is talking about multiple pairs and say 'These pairs ARE smaller than those pairs'.