The correct answer is option 'D', the Indus Valley Civilization.

The Indus Valley Civilization was one of the earliest urban civilizations in the world, existing around 2500 BCE to 1900 BCE. It was located in what is now modern-day Pakistan and northwestern India. The civilization developed an advanced system of urban planning, with organized cities, standardized weights and measures, and a drainage system. They had a system of writing as well, with thousands of inscriptions found on seals, pottery, and other artifacts.

However, despite the extensive archaeological findings, the script of the Indus Valley Civilization has not been deciphered yet. This has posed a great challenge to researchers and scholars who have been trying to understand the language and communication system of this ancient civilization.

Here are some possible reasons why the Indus Valley script remains undeciphered:

1. Lack of Bilingual Texts: One of the main challenges in deciphering ancient scripts is the presence of bilingual texts, where the same text is written in two different languages. This allows researchers to compare the known language with the unknown one and decipher the script. Unfortunately, no such bilingual texts have been found in the Indus Valley Civilization.

2. Limited Number of Inscriptions: While thousands of inscriptions have been discovered, the number of unique symbols used in the script is relatively small. This makes it difficult to identify individual letters or words, as there are not enough examples to establish patterns or context.

3. Absence of a Rosetta Stone: The Rosetta Stone was instrumental in deciphering the ancient Egyptian hieroglyphs. It contained the same text in three scripts: Egyptian hieroglyphs, Demotic script, and ancient Greek. Unfortunately, no similar artifact has been found in the Indus Valley Civilization, which could provide a key to understanding the script.

4. Complexity of the Script: The Indus Valley script is considered to be quite complex, with over 400 different signs. This complexity adds to the difficulty in deciphering the script, as it may require a combination of phonetic, ideographic, and symbolic elements.

5. Lack of Decoding Methods: Despite numerous attempts over the years, researchers have not yet developed a reliable method to decipher the script. Various approaches have been proposed, including linguistic, statistical, and symbolic methods, but none have been successful in providing a definitive translation.

In conclusion, the script of the Indus Valley Civilization remains undeciphered due to the lack of bilingual texts, limited number of inscriptions, absence of a Rosetta Stone-like artifact, complexity of the script, and the lack of successful decoding methods. Further research and new discoveries may eventually lead to the decipherment of this ancient script, providing valuable insights into the language and culture of the Indus Valley Civilization.