The way the sentence is phrased can be interpreted in various ways.
- Tom and the twins have completed an audition and they are deemed to not be good enough to perform at an upcoming concert.
- The writer simply assumes that Tom and the twins are not good enough to sing at the concert.
For the first interpretation. The audition would have concluded so the verb should be in the past tense. The past tense of sing is “sang” not “sung”. “Sung” is past participle tense which is used after a helping verb, usually “has”, have” or “had”, so it would be incorrect. Therefore, the first interpretation would not work since there is no correct past tense in the multiple choices.
For the second interpretation, the writer is assuming at present that that neither Tom nor the twins sing well enough. Therefore, “sing” is correct.
*Note: For connectors such as either/or, neither/nor, in addition to, as well as, not only/but also, we always look at the one at the back (nearer to the verb).