To find the missing number in the sequence 2, 6, 15, 28, 55, 78, we need to analyze the pattern.
If we calculate the differences between consecutive terms, we get the sequence 4, 9, 13, 27, 23.
The difference between the first and second terms is 4, the second and third terms is 9, and so on.
Looking at these differences, we notice that they are increasing by 5 each time.
Now, let's add these differences to the last number in the original sequence:
78 + 4 = 82
82 + 9 = 91
91 + 13 = 104
104 + 27 = 131
131 + 23 = 154
Therefore, the missing number in the sequence is 154.
Out of the given options, the correct answer is D) 119.