To determine which is an invalid IP address from the given options, we need to understand that an IPv4 address is composed of four numbers separated by dots, where each number must be between 0 and 255 inclusive.

1. The first option, 201.34.50.1:
   • All segments are within the valid range of 0 to 255.
2. The second option, 10.0.0.1:
   • All segments are between the valid range of 0 to 255.
3. The third option, 192.0.23.204:
   • All segments are within the valid range of 0 to 255.
4. The fourth option, 91.4.5.256:
   • The last segment is 256, which is outside the valid range of 0 to 255.

Therefore, the only invalid address among the options is 91.4.5.256 because the value '256' is not valid in any segment of an IPv4 address.

This makes 91.4.5.256 the correct answer.