What is this structure in human brain?

Short answer
Based on shape and approximate position, I think it is the corpus callosum.

Background
I think it is the corpus callosum (Fig. 1). The corpus callosum is approximately 10 cm in length and is C-shaped. It becomes thicker posteriorly, as is also evident in your image. The corpus callosum is a structure consisting of white-matter, containing about 200 million axons.

^{Fig. 1. Mid-saggital section of the brain showing the corpus callosum. source: University of Central Florida}

I think it is not the caudate nucleus, alhough it has also the curved shape. The caudate is positioned more laterally than the corpus callosum and by the looks of it your picture shows a mid-sagittal section of the brain. The caudate seems to be positioned too far laterally in the brain as far as I can see (Fig. 2).

^{Fig.2. Nucleus caudatus. source: Brain Notes}