What feature of Part D plans may vary for beneficiaries based on income levels?

Part D plans, which provide prescription drug coverage under Medicare, can have varying monthly premium amounts based on beneficiaries' income levels. Specifically, individuals with higher income can be subject to an Income Related Monthly Adjustment Amount (IRMAA), which increases their monthly premium for Part D coverage. This adjustment is designed to ensure that those who have a higher financial capability contribute a fairer share towards their healthcare costs.

In contrast, the coverage provided, the availability of providers, and the level of co-insurance generally remain consistent across beneficiaries regardless of income. While certain benefits, like specific medications or treatments, may vary between different Part D plans, they do not specifically change based on a beneficiary's income level. Therefore, the key feature influenced by income within Part D plans is indeed the monthly premium amount, which can differ significantly for those with higher earnings.