Chapter 13 - Partial Derivatives - 13.1 Functions Of Two Or More Variables - Exercises Set 13.1 - Page 915: 31

False.

We are given that \[ \sqrt{1-x^{2}-y^{2}}=f(x, y, z) \] We know that the function $\sqrt{q}$ has the domain $q \geq 0$ So here, it should be \[ \begin{array}{l} 1-x^{2}-y^{2} \geq 0 \\ \Rightarrow x^{2}+y^{2} \leq 1 \end{array} \] So, it denotes a circle or cylinder in 3D space which has a radius less than or equal to 1. Thus, it is not a disk.