Which type of externality does not consider the costs to others?

D. Positive

Externalities are economic side effects that affect a third party not directly involved in a transaction.

• Negative externalities occur when an activity imposes a cost on a third party (e.g., pollution). These do involve costs to others.
• Positive externalities occur when an activity provides a benefit to a third party (e.g., vaccination, education). These do not impose costs on others; instead, they provide benefits.

The question asks which type of externality "does not consider the costs to others." This means the externality itself is not about imposing costs on others. Positive externalities fit this description because their nature is to provide benefits, not costs, to third parties.

The final answer is $\text{D}$.