Explanation:

Researchers from the Indian Institute of Technology-Guwahati (IIT-G) and the Bose Institute, Kolkata have developed an advanced injectable hydrogel for localised cancer treatment.

• Statement 1 is correct: Hydrogels are water-based, three-dimensional polymer networks capable of absorbing and retaining fluids. Their unique structure mimics living tissues, making them suitable for biomedical applications.

• The hydrogel, composed of ultra-short peptides — biocompatible and biodegradable building blocks of proteins — is designed to remain insoluble in biological fluids, ensuring it stays localised at the injection site. 

• It responds to elevated glutathione (GSH) levels, a molecule abundant in tumour cells.

• Upon encountering high GSH levels, the hydrogel triggers a controlled drug release directly into the tumour, minimising its interaction with healthy tissues and reducing systemic side effects.

• Statement 2 is correct: They serve as a stable reservoir for anti-cancer drugs, releasing it in a controlled manner while sparing healthy cells from harm.

• Statement 3  is correct: The hydrogel remained localised at the tumour site, steadily releasing the drug over time without causing detectable side effects on other organs.

Hence option 4th is correct.