To determine the number of connections for an application running on App Engine that is accessed globally from various device types, the correct metric to use in Stackdriver Monitoring depends on the type of connections you are interested in. Option A, 'flex/connections/current', is not a valid metric for App Engine. Option B, 'tcp_ssl_proxy/new_connections', tracks the number of new TCP or SSL proxy connections, which might not give the total number of current connections. Option C, 'tcp_ssl_proxy/open_connections', is the correct metric as it provides the number of currently open TCP or SSL proxy connections, which is what you need to know for monitoring active connections. Option D, 'flex/instance/connections/current', is specific to App Engine Flex and not applicable for standard App Engine or the scenario described. Therefore, the correct answer is C.