Which part of a reflex arc connects the effector to the spinal cord?

The sensory neuron

The motor neuron

The receptor

The reflexor

I think it is the motor neuron, but I am not sure.

In a reflex arc, it is the sensory neuron that is directly connected to the motor neuron in the spinal cord. The sensory neuron is the one that carry the message from the receptor to the central nervous system which is the spinal cord and brain. While a motor neuron is the one that carries the message from central nervous system to the effector.

so the answer is motor neuron, and you're correct.