Final answer:
The $50,000 gain that Serena was not able to exclude from the sale of her principal residence is classified as C) long-term capital gain.
Step-by-step explanation:
The $50,000 gain that Serena was not able to exclude from the sale of her principal residence is classified as Long-term capital gain.
When a taxpayer sells their principal residence, they may be eligible to exclude up to $250,000 of the gain if single, or up to $500,000 if married filing jointly. However, any gain above that exclusion amount is subject to taxation.
In Serena's case, she had a gain of $300,000 but was only able to exclude $250,000. The remaining $50,000 is considered long-term capital gain and will be subject to taxation at the appropriate capital gains tax rate.