active to passive: They should be auditing the subsidiary accounts this quarter.

Explanation:

Step 1: Identify the tense

Active sentence:
👉 They should be auditing the subsidiary accounts this quarter.

• “should be auditing” → Modal + Continuous form
• Structure: Modal (should) + be + V1 + ing

Step 2: Passive form rule

👉 Passive of:
Modal + be + V-ing
➡️ Modal + be + being + V3

✔️ Apply the rule:

• Object → Subject: the subsidiary accounts
• Verb: should be auditing → should be being audited

✔️ Final sentence:

The subsidiary accounts should be being audited by them this quarter.

❌ Why other options are wrong?

1. should have been audited ❌
   → Perfect form (changes meaning)
2. are being audited ❌
   → Present Continuous (no “should”)
3. should be audited ❌
   → Simple form (loses continuous sense)

💡 Important Concept:

👉 “should be being audited” is grammatically correct but rarely used in daily English
👉 In real usage, people often simplify:
✔️ The subsidiary accounts should be audited this quarter.

Correct answer:

👉 the subsidiary accounts should be being audited by them this quarter.