**Step 1: Identify the relevant industry**
Based on the provided facts, Acea S.p.A. is an Italian multi-utility operating in the electricity, gas, and water sectors. Therefore, the relevant industry methodology is **Regulated Utilities**.

**Step 2: Estimate 2021 Adjusted_EBITDA**
Formula: Adjusted_EBITDA = EBITDA (reported or reconstructed) + adjustment_leases + nonrecurring_losses - nonrecurring_gains ± pension_adjustments ± joint_venture_proportional_EBITDA ± other_normalization_adjustments
- Reported EBITDA (Gross Profit + Impairment Loss + Amortization) for 2021:
  Gross Profit: 1,256,075,000 EUR
  Impairment Loss (IFRS9): 86,207,000 EUR
  Amortization: 588,768,000 EUR
  EBITDA = 1,256,075,000 + 86,207,000 + 588,768,000 = 1,931,050,000 EUR
- Adjustment for leases: Operating lease interest/repayments are not explicitly broken out from operating expenses; standard adjustment is 0.
- Pension adjustments / JV proportional / Non-recurring: 0 (no data provided for these specific adjustments).
- **2021 Adjusted_EBITDA = 1,931,050,000 EUR**

**Step 3: Estimate 2021 FFO**
Formula: FFO = Adjusted_EBITDA - cash_interest - cash_taxes
- Cash Interest: Finance Costs for 2021 = 97,388,000 EUR (used as proxy for cash interest)
- Cash Taxes: Income Taxes Paid for 2021 = 180,117,000 EUR
- FFO = 1,931,050,000 - 97,388,000 - 180,117,000 = 1,653,545,000 EUR

**Step 4: Estimate 2021 Adjusted_Debt**
Formula: Adjusted_Debt = (reported_debt + leases + pension_deficit + guarantees + hybrid_debt_portion + other_debt_like_items) - eligible_cash
- Reported Debt: Other Noncurrent Financial Liabilities (4,791,979,000) + Other Current Financial Liabilities (285,222,000) = 5,077,201,000 EUR
- Leases: Right-of-use Assets are reported (53,096,000), but lease liabilities are not explicitly broken out from payables; we use 0.
- Eligible Cash: Cash and Cash Equivalents = 680,820,000 EUR
- Adjusted_Debt = 5,077,201,000 - 680,820,000 = 4,396,381,000 EUR

**Step 5: Calculate 2021 Adjusted_Debt / Adjusted_EBITDA**
- Ratio = 4,396,381,000 / 1,931,050,000 = 2.28x

**Step 6: Calculate 2021 FFO / Adjusted_Debt**
- Ratio = 1,653,545,000 / 4,396,381,000 = 0.376 (37.6%)

**Step 7: Estimate 2022 Adjusted_EBITDA**
- Gross Profit: 1,305,021,000 EUR
- Impairment Loss: 113,370,000 EUR
- Amortization: 625,799,000 EUR
- EBITDA = 1,305,021,000 + 113,370,000 + 625,799,000 = 2,044,190,000 EUR
- **2022 Adjusted_EBITDA = 2,044,190,000 EUR**

**Step 8: Estimate 2022 FFO**
- Cash Interest: Finance Costs for 2022 = 111,670,000 EUR
- Cash Taxes: Income Taxes Paid for 2022 = 178,506,000 EUR
- FFO = 2,044,190,000 - 111,670,000 - 178,506,000 = 1,754,014,000 EUR

**Step 9: Estimate 2022 Adjusted_Debt**
- Reported Debt: Other Noncurrent Financial Liabilities (4,722,263,000) + Other Current Financial Liabilities (619,418,000) = 5,341,681,000 EUR
- Eligible Cash: Cash and Cash Equivalents = 559,908,000 EUR
- Adjusted_Debt = 5,341,681,000 - 559,908,000 = 4,781,773,000 EUR

**Step 10: Calculate 2022 Adjusted_Debt / Adjusted_EBITDA**
- Ratio = 4,781,773,000 / 2,044,190,000 = 2.34x

**Step 11: Calculate 2022 FFO / Adjusted_Debt**
- Ratio = 1,754,014,000 / 4,781,773,000 = 0.367 (36.7%)

**Step 12: Classify the year-on-year credit trend**
- **Signal 1 (Leverage):** Change in Adjusted_Debt / Adjusted_EBITDA = 2.34x - 2.28x = +0.06x. 
  Threshold is ≥ +0.2x for Deteriorating. Since +0.06x is between -0.2x and +0.2x, Signal 1 is **Stable**.
- **Signal 2 (Cash-flow coverage):** Change in FFO / Adjusted_Debt = 0.367 - 0.376 = -0.009. 
  Threshold is ≤ -0.03 for Deteriorating. Since -0.009 is between -0.03 and +0.03, Signal 2 is **Stable**.
- **Combining the signals:** Both signals agree on Stable.

<answer>Stable</answer>