It can be noted that the sentence is in the superlative degree, signifying that the book that is mentioned in the sentence is ultimate amongst all the books the subject has read so far. So, the superlative adjective "most interesting" is appropriate for comparing the book to all others read, indicating it is the best in terms of interest.
The complete sentence: This is the most interesting book that I have read till now.
Hence, the correct answer is option (b).
Note:
· The Positive Degree is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made.
· The Comparative degree denotes a higher degree of quality than the positive and is used when two things are compared.
· The Superlative degree indicates the highest degree of quality and is used when more than two things (or sets of things) are compared.