describes the contributions of Charles Martel to Europe during the Middle Ages
He united the Franks to fight off Muslim invasion.
He united eastern and western portions of the Roman Empire.
He encouraged the development of the arts.
He spread Christianity to new lands.
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I'd go with the first choice, that he united the Franks to fight off Muslim invasion. At the Battle of Tours in 732, he stopped the Moorish advance through what is today Spain, breaking the spearhead of their invasion and successfully ending their efforts at invasion. He earned the nickname "the Hammer" because of the way he beat back his enemies with his armies.
If it weren't for that victory, it's quite possible that the Moors could have continued on into the heart of Europe, crushing the Christian kingdoms there.
In light of that, the other choices don't make as much sense, and he certainly didn't unite the Eastern and Western parts of the Roman Empire, either.