Find the missing number?

Answer: Option B

Start with the numbers along the top right diagonal of the diagram, and add pairs of adjacent numbers together, putting the result in the square to the lower left of the original pair of numbers. Continue in this way towards the lower left hand corner. I.e
4 + 2 = 6
2 + 1 = 3
6 + 3 = 9
So,

1 + x = 7
x = 6