Determining Voltage and DC/AC from Wiring Diagram for Spark Door Buzzer
simulate this circuit &ndash; Schematic created using CircuitLab

As noted in comments, the transistor can be any general purpose switching transistor. The transistor must be able to handle the current necessary to activate the relay coil. The relay coil must be able to be switched on by 3.3V. The relay must be capable of switching a 16 VAC load and be able to handle 0.625 Amps.

Note that the AC ground likely won't be connected to the Spark Core's ground because there shouldn't be a need and it could cause issues if you hook things up wrong.

A high signal from the Spark Core will activate the relay and activate the Buzzer. A low signal would block current in Q1 which would prevent RLY1 from activating which would prevent the buzzer from activating.