**2019:**

The electric flux through a closed surface is given by:

```
Φ = q/ε0
```

In this case, the charge is at the centre of the cube, so the electric flux through any face of the cube is:

```
Φ = Q/6ε0
```

**2018:**

The electric field at a point due to a charge q is given by:

```
E = kq/r2
```

In this case, the point C is the centroid of the triangle, so the distance from the charge q to the point C is:

```
r = a/√3
```

Therefore, the electric field at the point C is:

```
E = (q/4πε0a2)√3
```

**2017:**

The electric flux through a closed surface is given by:

```
Φ = q/ε0
```

In this case, the charge is at the centre of the square, so the electric flux through the square is:

```
Φ = q/4πε0
```