Who was Marco Polo?

An explorer (or traveller) is someone whose historical significance rests on undertaking a journey into distant or unfamiliar regions to discover and document new geography, peoples, trade routes, and resources -- distinct from a scientist (who investigates natural phenomena through systematic experiment), a writer (whose primary vocation is composing original literary works), or a publisher (who produces and distributes written works).

Marco Polo (c. 1254 to 1324) was a Venetian merchant who journeyed overland along the Silk Road to the Mongol-ruled court of Kublai Khan in China, spending roughly two decades travelling through Central Asia, China, and Southeast Asia before returning to Venice. This journey, not any experiment or literary career, is what defines his historical identity: leading reference works (for example, Encyclopaedia Britannica's biography is titled "Marco Polo -- Explorer, Venetian, Silk Road") identify him first and foremost as an explorer.

Why the other options do not fit as well:

• Scientist: describes someone who conducts systematic experiments or investigation of natural phenomena (for example, Galileo). Marco Polo's fame rests on his overland journey and geographic accounts, not on experimental or scientific investigation.

• Writer: after his return, Marco Polo dictated an account of his travels while imprisoned in Genoa, and it was set down in writing by a fellow prisoner, Rustichello da Pisa, a romance writer by profession. Because Marco Polo supplied the account rather than composing it as a professional author, and because his own historical identity is defined by the journey itself, "explorer" remains the standard characterization even though some references also mention his connection to the resulting book.

• Publisher: describes a person or enterprise that produces and distributes written works. Marco Polo neither produced nor distributed any book; this term does not describe his actual activity at all.

The keyed option, "An explorer," is therefore the standard and best-supported characterization of Marco Polo among the options offered.