Which subnet does host 131.50.39.23/21 belong to?

The /21 subnet mask has subnets in multiples of 8. So the networks would be 131.50.8.0/21, 131.50.16.0/21, 131.50.24.0/21, 131.50.32.0/21, and 131.50.40.0/21. The IP address of 131.50.39.23/21 would belong to the 131.50.32.0/21 network with a valid range of 131.50.32.1 to 131.50.39.254. Therefore, the network 131.50.39.0/21 cannot be a network ID because it belongs to the 131.50.32.0/21 network. Both the 131.50.16.0/21 and 131.50.8.0/21 network IDs are outside of the range for the host used in this question.