Such questions have always patterns. they are called series or sequence. for those who know how to solve this they call it common sense. May be you are finding some difficulty otherwise they are very easy with some practice.
You have to find some patterns. some rule on which next term is written and using that pattern you have to find the answer.

like if it is given. 2, 4, 6 the next term will surely be 8.
and if it is. 3, 6, 9 next term would surely be 12. such sequences are a bit more complex than these i told. usually 3 terms need to be given to find the fourth. because two terms cant tell you about any pattern.

As, if it is 3 , 6, here you dont know whats the pattern. whether they doubled 3 to get 6 or simply added 3 in 3. if the doubled 3 then 6 would also be doubled to get 12 and series will be 3, 6,12, 24 and if they added 3 in 3 then it would be. 3,6 ,9,12,15 etc..
So mostly at least 3 terms are given and sometimes 4 if pattern is more complex.

Anyway in short you have to find common pattern in those 3 or 4 given terms. its like finding the relation between the terms.

The relation or pattern which exists between 1st and 2nd term and if the same exists in 2nd and 3rd term then this must exist in 3rd and 4th term.

In 2 terms like 3, 6 you cant find pattern. because it may be a single thing while patterns means repetition of something. If you find that pattern or rule or relation by which first 3 terms are written you are sure to find the fourth term even if the option is not given.

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"The only necessary thing for the triumph of evil is for good men to do nothing."
--Edmund Burke (1729-1797)