The pronoun “their” (L.36) refers to “clients’ computer syst...

"Their" in l.36 refers back to Accountants and auditors. As simple as it gets.

O pronome their e anafórico, ou seja, remete e a termo já mencionado. No texto, fica claro que o referente é accountants and auditors, portanto, errada a questão.

refere-se a accountants and auditors

Errado.

Texto Computers enable accountants and auditors to be more mobile and to use their clients’ computer systems to extract information from databases and the Internet.

Questão The pronoun “their” (l.36) refers to “clients’ computer systems” (l.37). 

No texto: their clients’ computer systems: sistemas informatizados de seus clientes (clientes dos contadores e auditores)

Obs:

Questão comentada pelo Prof.  Alexandre Hartmann