Model comparison question

[MobiusStrip]

There are two models that both make different predictions for value x. The first model, M_1 has no parameters and x=0, whereas M_2 has one unspecified parameter, a, and predicts that x=a. Then, a sampling distribution of the data is determined, which is P(d|x,σ) = N(d;x,σ^2) = 1/(2πσ^2)^1/2 * exp[-1/2 * (d-x/σ)^2].

How can I determine if the two models, M_1 and M_2, are fully specified by the information given above?