Find the error.... English

Question

The only good (a) /about these pens (b) /are their colour and there size (c) /no error

Answer 1

The only good thing/ about these pens(b) / are their colour and their size.(c)/ No error

Answer

(c) : ‘Is’ in place of ‘are’. The subject ‘the only good thing ‘ is singular and thus the verb will be singular.

Answer 2

(c) : ‘Is’ in place of ‘are’. The subject ‘the only good thing ‘ is singular and thus the verb will be singular.