Leak from the pipe in the basement that connects to kitchen sink
Your drain might be backing up , a 1-1/2 or 2” sink dumping into a 4” drain should not have any back pressure . I see your arrows but can not tell why it would leak there , I would try a silicone sealer . If there is not a vent close to the sink this could cause issues but at the point where the size jumps would be a new one for me. If you run water for a few minutes will the leak get worse?  How long after turning off the water will that spot leak?  If it leaks for a while after stopping the water flow I would want to run a snake down the drain and see if it is backing up. 

Added per op request;
On fernco or no hub rubber connectors I regularly use silicone sealant (cast iron and concrete always) I would loosen the pipe clamps move the hub and clean it the I would add some silicone and retighten the clamps.  This will help seal the rubber for the life of the coupling.