Pretend you hold a large stake in Novo Nordisk $($which makes the weight$-$loss drugs Wegovy and Ozempic$)$ and you do not intend to sell that stake. You have other money to invest and want to diversify. Which of the following investments provides the worst diversification?

a$.$ S&P $500$ index fund

b$.$ Portfolio of pharmeceutical stocks in firms that are in late$-$stage development of drugs that will compete with Ozempic and Wegovy

c$.$ Portfolio of REITS focused on multi$-$family residential in the UK