Hey! Option (b) is correct.
Reason: GDP measures the total value of all final goods and services in an economy. Here, Z is an intermediate good, used for the production of X and Y. Hence, it wont be counted in the GDP.
Hey! According to me , the value of z as an intermediate good is being counted twice coz it is used in the production of both x and y so the final value of x also includes it and the final value of y also includes it.. Isn't this double counting the value of z ? And hence we should subtract it once . Please revert.