The answer is B: 22.

The rule
Add 1, then add 2, then add 3, then 4… The gap grows by one every time.
The pattern as Python code
Here is a tiny Python program that follows the same rule. Type it into IDLE and run it to print the whole pattern, including the answer.
number = 1
gap = 1
for step in range(7):
print(number)
number = number + gap
gap = gap + 1
When you run it, Python prints:
1 2 4 7 11 16 22
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!