At the edge of the grassland,there is a sign which says,“( )the grass”

keep back 意为：退缩; 隐瞒; 留下; 阻止。而题意是那个示牌写着“不要靠近草地”，故应选C，keep off避开; 不(让)接近。B的keep away 后应跟from

这里根据句意就可判断出不是“阻止草地”，而是“远离草地”啊。