Which of the following species of heart wood is not recommended for external use?

When considering the properties of wood species for external use, it's important to evaluate their natural durability against decay and insect attack. Fir, while strong and commonly used in construction, is not naturally resistant to weather elements and pests when compared to other species. This makes it less suitable for outdoor applications without significant treatment or protective measures.

In contrast, oak is naturally durable, especially white oak, which has good water resistance. Cherry, although it can be used for outdoor projects, is typically favored for its aesthetic in interior applications due to its susceptibility to color change and deterioration when exposed to the elements. Redwood is well-known for its exceptional resistance to decay and insects, making it an excellent choice for outdoor use. Thus, fir is the least recommended for external applications among the options provided, highlighting the significance of selecting the right wood species based on their performance characteristics in exposed conditions.