I don't know how to find answer to this question. The answer is given as option (D).
I know that total degrees of freedom is 18-1 = 17 and degrees of freedom for factor $\alpha $ and $\beta$ are 1 and 3 respectively. But does the additional constraints in the problem affect the DF? It would be helpful if someone can provide some materials related to this. I can't find anything in the internet.