The correct answer is (C).
In this series there is an object with three numbers and an object with four numbers alternately. There are four numbers in the last object in the question which means the next object should have three numbers.
In addition there is an object with a square in it and an object with a triangle in it alternately. There is a triangle in the last object in the question, which means the next object should have a square in it.
There is also a third feature in this series, the sum of numbers get smaller in two as the series goes on. The sum of the last object is six so the sum of the numbers in the next object should be four.
(C) is the only object that fits these features.
How to solve this question:
There are many features that you need to pay attention to. You can start with the one that stands out the most - the presence of a triangle and a square alternately. The last object in the given series is a triangle, so the next object should have a square in it. If you are looking for an object with a square in it, you are left with options (A), (C) and (E). Now you need to understand the meaning of the numbers. You can check different features that might be relevant - the sum of the numbers, the amount of numbers, order of numbers, and type of numbers (even or odd).
In this question, you need to calculate the sum of the numbers, what gives you a clue about calculating the sum of numbers in the objects of the question itself. You can see that the sum of numbers gets smaller by two as the series goes on, and the sum in the last object of the given series is six so you need to look for an object where the sum of the numbers is four. You are left with options (C) and (E). Then you need to understand the difference between them. You can see there is a difference in the amount of numbers. You can then examine whether this difference is relevant and discover that in the question there is an object with three numbers and an object with four numbers that change alternately. The last object in the given series has four numbers, meaning the next object should have three numbers. Answer (C) fits this rule.