This question examines the positional relationship between two circles, in order to prevent sheep from eating vegetables, that is, draw a circle with A as the center and the length of the rope as the radius, which does not intersect with the vegetable garden.

First, we need to calculate: the length of OA.

Can connect OA, δδAOB is RT δ, OB=6, AB=8, so AO= 10,

The longest rope length for tying sheep: 10-6=4m. Only a is less than 4 in this question, so choose a.