Explanation

A decrease in competition amongst market makers for the ETF most likely reduces an ETF's bid-ask spread.

Key Points:

• Competition among market makers drives tighter spreads as market makers compete to offer better prices
• When there are more market makers competing, they are forced to offer narrower bid-ask spreads to attract order flow
• A decrease in competition would allow remaining market makers to widen spreads and increase their profit margins

Why other options are incorrect:

• Brokerage fees: These are separate transaction costs that don't directly affect the bid-ask spread
• Daily share volume: Higher volume typically reduces spreads, so a decrease in volume would likely increase spreads

Therefore, reduced competition among market makers allows them to maintain wider spreads, benefiting their profitability at the expense of investors.