Veolia Environnement shows a combination of high capital‑expenditure intensity (≈ €2.8 bn per year), a large acquisition (the Suez deal) that has already added a substantial amount of goodwill and intangible assets, and a net‑debt/EBITDA ratio around 4× – placing the company under notable leverage pressure. Its short‑term refinancing needs are significant (≈ €6.5 bn of current financial liabilities) and the rating is under pressure to preserve investment‑grade status.  

Given these factors, the situation aligns most closely with the “high capex intensity OR large acquisition pipeline” scenario described in the guidance (11.25% tier). Issuing enough hybrid securities to bring the hybrid proportion to about 11.25 % of total adjusted capital (≈ €5.1 bn hybrid, i.e., an additional ≈ €1.6 bn, well within the €3 bn annual limit) would provide meaningful leverage relief while only marginally raising the cost of debt, helping to protect the rating without approaching the statutory 15 % cap.  

Therefore, the recommended level of hybrid bond utilization for the next 18 months is 11.25 % of total adjusted capital.

<answer>11.25%</answer>