Which of the following is the correct way to set a background color of blue to an HTML element with id = "color_change"?

a.
```
document.getElementById("color_change").style.background = "blue";
```

b.
```
document.getElementById("color_change").innerHTML = "blue";
```

c.
```

```
document.getElementById("color_change").style = blue(this.id);

d.
```
document.getElementById("color_change").this.id = background("blue");
```

a.
```
document.getElementById("color_change").style.background = "blue";
```