The most compelling reason from your options for learning Spanish is C: because speaking other languages helps you better understand other people and cultures.
Learning Spanish not only allows you to communicate with a large number of people globally but also deepens your appreciation of diverse cultures, traditions, and perspectives. This cultural understanding can enhance your personal and professional relationships and broaden your worldview.
While A (developing better artistic skills) could be a potential benefit if you're interested in Spanish art and literature, it is not a primary reason for learning the language. Similarly, B (impressing family and friends) might be a secondary motivation but doesn't capture the deeper significance of language learning in fostering understanding and connection.