What is the minimum number of virtual NICs that you can configure for a virtual machine?

Four is not a minimum value but rather a count that approaches typical per-VM NIC limits, which is not relevant to the minimum.

One is incorrect because vSphere does not mandate that any network adapter be attached to a VM for it to be a valid configuration.

Virtual NICs are optional components in a VM configuration. A VM can be created and powered on without any network adapter assigned, meaning zero is a valid and supported minimum. Network adapters can be added later as needed without requiring one at creation time.

Two is incorrect for the same reason as one - no minimum NIC count is enforced by the hypervisor.