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Before starting let us see what is the composite number?
"A number that can be divided exactly by numbers other than 1 or itself is called a composite number".
So according to the defination of composite number are 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, ....
Now the question is what is the largest composite number less than 20 ? And given four option are 17,18,19,20.
Here 20 will not be included because we need less then 20.
Here 19 also will not be included because 19 is not composite number it is a prime number.
Here 18 is composite number also less then 20. So 18 is the required composite number.
Here 17 is also prime number?
So final answer is 18 is the required composite number which is also less then 20.