The correct answer is A. NLTK.
NLTK is a free, open-source natural language processing library for the Python programming language. It supports a wide range of natural language processing tasks, including text tokenization, part-of-speech tagging, named entity recognition, and sentiment analysis.
Pandas is a Python library for data analysis. It provides high-performance, easy-to-use data structures and data analysis tools for working with structured (tabular, multidimensional, potentially heterogeneous) and time series data.
Seaborn is a Python data visualization library based on Matplotlib. It provides a high-level interface for drawing attractive statistical graphics.
Scikit-learn is a Python machine learning library that features various classification, regression and clustering algorithms including support vector machines, random forests, gradient boosting, k-means and DBSCAN, and is designed to interoperate with the Python numerical and scientific libraries NumPy and SciPy.
In conclusion, NLTK is the most appropriate library for natural language processing tasks in Python.