Positive externalities can be defined as:

Positive externalities refer to the benefits that third parties receive from an economic activity or transaction that they did not directly participate in. For example, when a homeowner invests in beautifully landscaping their garden, neighbors may enjoy an aesthetically pleasing view without having contributed to the cost of the landscaping. In this case, the homeowner's investment spills over into benefits for the community, such as increased property values or enhanced public enjoyment of green space.

This definition highlights that positive externalities create extra advantages beyond the primary economic actors involved, leading to a situation where societal gains exist beyond what the market transactions might initially indicate.

The other options do not capture the essence of positive externalities. Costs incurred by third parties refer to negative externalities, which are detrimental impacts rather than benefits. Negative impacts on the economy also relate to adverse effects, and unforeseen costs in production similarly do not pertain to the concept of benefits, as they imply unintended financial burdens. Thus, the correct choice accurately reflects the definition and nature of positive externalities in economics.