Which metropolitan rail system became the first metro in the world to be certified as one hundred per cent green?

Introduction / Context:
Environmental certification of public transport systems shows a city commitment to sustainable development. A metro network that is certified as one hundred per cent green meets strict standards related to energy use, construction materials, and environmental impact. This question asks which metro system gained recognition as the first in the world to be certified fully green.

Given Data / Assumptions:

The subject is a metropolitan rail or metro system.
The system received certification as one hundred per cent green.
This is described as a world first in terms of green certification for metros.
Four Indian metro systems are listed: Hyderabad, Mumbai, Kolkata, and Delhi.

Concept / Approach:
The concept is awareness of sustainable infrastructure initiatives. Delhi Metro, managed by the Delhi Metro Rail Corporation (DMRC), is widely recognised for incorporating solar panels, regenerative braking, and energy efficient design. It received green certification for all its operations and projects, and was reported as the first metro in the world to achieve one hundred per cent green status.

Step-by-Step Solution:
Step 1: Note that the question refers to a metro system that is fully certified as green. Step 2: Recall which Indian metro network has repeatedly been in the news for green ratings and sustainability, namely Delhi Metro. Step 3: From the available options, select Delhi as the metro that received this unique recognition.

Verification / Alternative check:
Reports from rating agencies and announcements from DMRC highlight that Delhi Metro achieved one hundred per cent green certification for all its operations, making it the first metro in the world to do so. These sources confirm that option D is correct, while the other metro systems had not yet reached this milestone at the time.

Why Other Options Are Wrong:
Hyderabad Metro is modern and energy efficient but was not the system reported as the first completely green metro, so option A is incorrect.
Mumbai Metro is still expanding and did not receive this particular global recognition at that time, making option B wrong.
Kolkata Metro is India oldest metro but was not certified as one hundred per cent green, so option C is incorrect.

Common Pitfalls:
Students may mix up different environmental awards and assume that any new metro, such as Hyderabad, must have the most advanced green features. Another pitfall is ignoring the phrase first metro in the world, which strongly points to a system that has been in operation long enough to implement comprehensive measures. Recognising Delhi Metro established reputation for sustainability helps to answer accurately.

Final Answer:
The first metro system in the world to be certified as one hundred per cent green was the Delhi Metro.