1. The problem asks for the minimum number of vertices (nodes) a binary tree should have. 2. A binary tree is a tree data structure where each node has at most two children. 3. The smallest binary tree consists of just one vertex, which is the root node. 4. Therefore, the minimum number of vertices in a binary tree is one. 5. This means options like two, three, or four vertices are not minimum requirements but possible sizes of binary trees. Final answer: A binary tree should have at least **one vertex**.