The social networking site used by professional and business people who want…

A professional social networking platform is a site whose core purpose is to help individuals build and maintain a network of business and career contacts — showcasing work experience, skills, and professional achievements, and connecting job-seekers, employers, and companies — as distinct from platforms built around personal socializing, entertainment, or private messaging.

Among the given platforms, LinkedIn is built specifically around this purpose: user profiles are structured as professional résumés (work history, skills, endorsements), and its core features — job postings, company pages, and industry networking groups — are all oriented toward business and career connections. This matches the platform described in the question as being used by 'professional and business people who want to connect with other professionals.'

• Facebook is built around personal profiles for friends and family, News Feed sharing, and general-interest groups — its primary orientation is personal/social rather than career-focused.

• MySpace is built around customizable personal profile pages, music, and artist/fan promotion — an entertainment- and music-oriented platform, not a career-networking one.

• WhatsApp is built around private one-to-one and group messaging (text, voice, video calls) rather than public profiles or professional networking at all.

So the correct answer is LinkedIn.