Which verb agrees with its subject? somebody at one of the cash registers trying to soothe a crying baby.
a. were
b. are
c. have been
d. is

Somebody at one of the cash registers is trying to soothe a crying baby.
so d is the correct option.

I believe the answer would be:
"Somebody at one of the cash registers is trying to soothe a crying baby"
Using 'were' does not agree with somebody because you must use 'was' to agree with somebody and this is not one of our choices. 
They were trying to soothe a crying baby would be correct but you have the word somebody...one person.''
Using the words 'have been' would require changing 'somebody' to maybe 'people' have been.
Remember 'were' would indicate in the past, but this sentence is talking about someone at a cash register at this very moment...not in the past.