Which verb agrees with its subject?
the singers or their agent arranging their tour.
a.
is
b.
were
c.
are
d.
have been

I think c is the best verb that agrees with its subject

The answer is a.) is because when you have a plural and a singular verb combined using "or" you use the last subject to pick the verb. So it would be "their agent 'is' arranging". 

  match the example to the term.   1. formal standard english: by the power vested in me as a minister of the gospel, i now pronounce you man and wife. what god has joined together, let not man put asunder.2. colloquial english:   congratulations, jim and sarah. we're very happy for you. you for inviting us3. technical text: to register online at smith's bridal, complete the form below and press "enter." (directions, instructions or explanation) 4. regional dialect y'all make a fine couple! i reckon you'll be happy. 5. informational text: the photographer said he would photoshop in a better picture of the uncooperative flower girl.6. jargon: the aforementioned parties, in entering into said agreement, hereby mutually swear that they have, to the best of their abilities, provided a truthful representation of their intent . .   (legal jargon)7. informal standard english: way to go, guys! you'll do great! (no conjunctions used)8. slang: hey, just chill out. (used by a particular profession or group)9. literary text: entreat me not to leave thee or to refrain from following after thee. for whither thou goest i will go (bible)