Yes, option d) "all of the above" is the correct answer. The reason there are still many species unknown to modern science is due to a combination of factors. First, many of these species are found in remote environments that are difficult to access. Exploring these remote areas requires significant resources, time, and expertise.
Second, smaller species often receive less attention than large organisms. The study of larger organisms, such as mammals and birds, tends to attract more attention from researchers and the general public. Consequently, smaller species, including many insects, plants, and microorganisms, remain understudied.
Lastly, cataloging all the species on Earth is an enormous task. The Earth is home to an estimated 8.7 million species, and scientists have only identified and described a fraction of them. Discovering, documenting, and properly classifying species is a complex process that requires interdisciplinary collaborations and years of research.
Therefore, a combination of these factors—remoteness, lack of attention to smaller species, and the vastness of the task—contributes to the presence of many unknown species in the scientific community.