$\text{a)}$  

$x-2\ne 0$  

$x\ne 2$  

$D=\mathbb{R}-\left\{2\right\}$      

---

$\text{b)}$  

$x^2-1\ne 0$  

$x^2\ne 1$  

$x\ne 1,x\ne -1$  

$D=\mathbb{R}-\left\{-1,1\right\}$  

---

$\text{c)}$  

$3x^2\ne 0$  

$x^2\ne 0$  

$x\ne 0$  

$D=\mathbb{R}-\left\{0\right\}$  

---

$\text{d)}$  

$x+4\ne 0$  

$x\ne -4$  

$D=\mathbb{R}-\left\{-4\right\}$  

---

$\text{e)}$  

$x^2-9\ne 0$  

$x^2\ne 9$  

$x\ne 3,x\ne -3$  

$D=\mathbb{R}-\left\{-3,3\right\}$  

---

$\text{f)}$  

$3-x^2\ne 0\text{i}x\ne 0$  

$3\ne x^2\text{i}x\ne 0$  

$x\ne \sqrt{3}\text{i}x\ne -\sqrt{3}\text{i}x\ne 0$  

$D=\mathbb{R}-\left\{-\sqrt{3},0,\sqrt{3}\right\}$