Answer:
Port 161
Step-by-step explanation:
SNMP stands for Simple Network Management Protocol. This protocol is used by SNMP agents to transfer information about managed devices to the SNMP manager.
Port 161 is the default User Datagram Protocol(UDP) port used by SNMP agents, that is SNMP agents listen for incoming data requests on port 161. Port 162 is used by SNMP managers to receive incoming notifications.
Port 20 and 21 are not relevant for SNMP traffic.