Let’s watch for myra lane and richard and ben and frances can be on the lookout for the others.

The correct placement of the commas is shown in this  b. Let’s watch for Myra, Lane, and Richard; and Ben and Frances can be on the lookout for the others. 

Here are the following choices:
a. Let’s watch for Myra, Lane, and Richard, and Ben and Frances can be on the lookout for the others. 
b. Let’s watch for Myra, Lane, and Richard; and Ben and Frances can be on the lookout for the others. 
c. Let’s watch for Myra, Lane, and Richard, and: Ben and Frances can be on the lookout for the others. 
d. Let’s watch for Myra, Lane, and Richard; and Ben, and Frances can be on the lookout for the others.