Question

their number is reducing rapidly (give tail tag /question tag)

Answer 1

their number is reducing rapidly, isn't it?

Answer 2

Given sentence:

Their number is reducing rapidly.

Sentence after adding appropriate question tag:

Their number is reducing rapidly, isn't it?

Additional information:

Question tag may be simply and compactly defined as a single phrase or group of words that is added at the end of a statement or sentence in order to and converts it to a question.

The British English call it "Question Tag" whereas on the other hand American English call it "Tag Question" or "Tail Question".

Question tags are generally common in familiar conversation and not in formal writing or conversation.