"Chemical substances that spread across the synaptic cleft and bind to receptors on the dendritic spines of another cell."

Step 1: Translate the question. The question asks: "Chemical substances that spread across the synaptic cleft and bind to receptors on the dendritic spines of another cell." Step 2: Analyze the options provided. أ) خلية عصبية (Nerve cell): This is the entire cell, not the chemical substance. ب) نواقل عصبية (Neurotransmitters): These are chemical messengers that transmit signals across a synapse from one neuron to another. They are released into the synaptic cleft and bind to receptors on the postsynaptic neuron. ج) الفعل المنعكس (Reflex action): This is an involuntary action, not a chemical substance. د) التشابك العصبي (Synapse): This is the junction between two nerve cells, not the chemical substance itself. Step 3: Determine the correct option. The description accurately defines neurotransmitters (نواقل عصبية), which are the chemical substances responsible for transmitting signals across the synaptic cleft by binding to receptors on the postsynaptic cell. The correct option is ب). ب) نواقل عصبية That's 2 down. 3 left today — send the next one.