I am the one who does it.

[tufguy]

1) Doctor Johnson is the one that takes care of my family.

2) Doctor Johnson is the one who takes care of my family.

3) I am the one who does it.

4) I am the one that does it.

Please check these sentences.

 

[emsr2d2]

In general, we use "who" to refer to people, so I recommend that you use 1 and 3. However, you'll hear "that" from many native speakers too.