Work through the clues. Gram-positive cocci plus catalase positivity points to staphylococci, and in a joint infection that means Staphylococcus aureus. The single decisive clue is the antibiotic susceptibility: cefoxitin is the laboratory marker for methicillin resistance. A cefoxitin-sensitive strain is MSSA - it still possesses normal penicillin-binding proteins and is not MRSA.
$\text{Catalase}(+)\ \text{GPC} \Rightarrow \textit{S. aureus};\quad \text{Cefoxitin}(S) \Rightarrow \text{MSSA}$
For MSSA the optimal therapy is a $\beta$-lactamase-stable anti-staphylococcal penicillin: cloxacillin (or flucloxacillin, nafcillin, oxacillin). These bind PBPs more avidly and clear MSSA faster than glycopeptides. Plain penicillin G fails because staphylococcal penicillinase hydrolyses it, while vancomycin and linezolid should be held back for genuinely resistant (cefoxitin-resistant) MRSA to limit toxicity and resistance.
\[\boxed{\text{Cloxacillin}}\]