The answer is B: 8 dots.

The rule
Each picture has 1 more dot than the one before.
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.
dots = 3
for step in range(6):
print(dots)
dots = dots + 1
When you run it, Python prints:
3 4 5 6 7 8
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!