A concentration gradient across the mitochondrial membrane is caused by
Uneven distribution of K+ ions across the membrane
Uneven distribution of Cl- ions across the membrane
Uneven thickness of membrane
The uneven distribution of H+ ions across the membrane
The uneven distribution of H+ ions across the mitochondrial membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.
Hence, the correct option is d.