The Harappan script, also known as the Indus script, has been a subject of great mystery and intrigue for scholars and researchers. It is called enigmatic because despite extensive efforts, scholars have not been able to decipher and read the script completely. The appropriate reason for this is option 'C' - scholars are not able to read the script.

Here are the details explaining the answer:

Lack of Decipherment:
The Harappan script remains undeciphered, and scholars have not been able to fully understand its meaning and language. This has led to the script being labeled as enigmatic. Various attempts have been made to decipher the script, but none have been successful in providing a definitive understanding of its content.

Lack of Bilingual Texts:
One of the major challenges in deciphering the Harappan script is the absence of bilingual texts. Bilingual texts, which contain inscriptions in a known language alongside the unknown script, can provide a key to understanding the script's grammar and vocabulary. However, no such texts have been discovered yet, making it difficult to make meaningful comparisons and translations.

Absence of the Rosetta Stone:
The Rosetta Stone played a crucial role in deciphering Egyptian hieroglyphs. It contained inscriptions in three scripts, including Greek, which was a known language. This provided a starting point for understanding and deciphering the hieroglyphs. In contrast, the Harappan script lacks a comparable key or reference point, further complicating the decipherment process.

Lack of Linguistic Continuity:
Another obstacle in deciphering the Harappan script is the absence of a linguistic continuity between the ancient Indus Valley civilization and modern languages. Unlike ancient Egyptian hieroglyphs, which had a direct linguistic link to the Coptic language, the Harappan script does not have any known descendants. This makes it challenging to establish linguistic connections and decipher its meaning.

Limited Corpus of Inscriptions:
The number of Harappan script inscriptions available for study is relatively small compared to other ancient scripts. This limited corpus makes it difficult to identify recurring patterns, grammatical structures, or establish a comprehensive understanding of the script's vocabulary.

In conclusion, the Harappan script is called enigmatic because scholars have not been able to read or decipher it fully. The lack of decipherment can be attributed to various factors such as the absence of bilingual texts, the lack of a Rosetta Stone-like key, the absence of linguistic continuity, and the limited corpus of inscriptions. These challenges have made the Harappan script one of the most enduring mysteries of ancient civilization.