Correct Answer & Explanation
Option C
Why this is correct:
1. **Rule for Modal Verbs in Passive Voice:**
$$\text{Subject} + \text{Modal (must)} + \text{Verb (base form)} + \text{Object}$$
*changes to:*
$$\text{Object} + \text{Modal (must)} + \mathbf{be} + \text{Past Participle } (V_3) + \text{by} + \text{Subject}$$
2. Applying the rule:
- **Active:** We (*Subject*) + must (*Modal*) + take care of (*Verb*) + all living species on Earth (*Object*).
- **Passive:** All living species on Earth (*Object*) + **must be** + **taken care of** (*$V_3$*) + **by us** (*Subject*).