The correct answer is option 'A' - RIPv1.

Explanation:
RIPv1 (Routing Information Protocol version 1) is the best choice when the network diameter is more than 17 hops. Here's why:

1. Limitations of RIPv1:
RIPv1 has a limitation of supporting a maximum hop count of 15. This means that it can only handle networks with a diameter of up to 15 hops. If the network diameter exceeds 15 hops, RIPv1 will not be able to accurately calculate and maintain the routing tables.

2. Network Diameter:
The network diameter refers to the maximum number of hops between any two nodes in the network. In this case, the network diameter is more than 17 hops, which exceeds the maximum hop count supported by RIPv1.

3. RIPv2:
RIPv2 (Routing Information Protocol version 2) is an enhanced version of RIPv1 that overcomes some of its limitations, including the maximum hop count. RIPv2 supports a maximum hop count of 16, which allows it to handle slightly larger networks compared to RIPv1.

4. EIGRP:
EIGRP (Enhanced Interior Gateway Routing Protocol) is another routing protocol that is commonly used in larger networks. Unlike RIPv1 and RIPv2, EIGRP is a distance vector protocol that uses more advanced algorithms to calculate routes and maintain routing tables. EIGRP can handle larger network diameters and provides faster convergence compared to RIPv1 and RIPv2.

5. Conclusion:
Considering the limitations of RIPv1, it is not suitable for networks with a diameter of more than 17 hops. RIPv2 or EIGRP would be better choices in such scenarios. However, since the given options only include RIPv1 and RIPv2, RIPv1 is the only suitable choice among them.

In summary, when the network diameter exceeds 17 hops, RIPv1 is the most appropriate protocol to select from the given options.