A) Although experienced, the ornithologists participating in the survey had never before visited the forests of the Kamchatka Peninsula
- not important. it does not suggests the ornithologist made a mistake.
B) The Banded Nuthatch was always a rare bird, and the last verified sighting prior to 1895 was in 1815
- this statement weakens the argument. It's always rare and the last time it was sighted in 1895 (when it is assumed to be extinct since then) was 80 years back in 1815 ! So the argument that the ornithologist made a mistake is flawed.
C) It is rare that a species thought to be extinct is found to be still in existence
- does not weaken the argument.
D) The Lesser Nuthatch, a bird known to be widespread in the forests of the Kamchatka Peninsula, is very similar to the Banded Nuthatch in appearance
- This strengthens the argument.
E) The ornithologists surveying the birds of the Kamchatka Peninsula are among the most respected and highly regarded in their field
- not important. Respected and highly regarded does not mean that are always right.
I'll go with (B)