Which one of the following materials is strongly paramagnetic due to the presence of unpaired f-electrons and is widely used in MRI contrast agents?

Gadolinium (Gd) is a lanthanide element with the atomic number 64. Its electronic configuration features seven unpaired electrons in the 4f subshell ([Xe] 4f⁷ 5d¹ 6s²), which is the maximum number of unpaired f-electrons possible. This unique electronic structure makes it strongly paramagnetic. In medical diagnostics, gadolinium-based contrast agents (GBCAs) are widely used in Magnetic Resonance Imaging (MRI). They function by interacting with water molecules in the body, shortening the T₁ relaxation time of nearby protons, which significantly enhances the clarity and contrast of the resulting images. In contrast, Lead and Bismuth are diamagnetic (repelled by magnetic fields), and Platinum is a d-block transition metal that does not possess the specific f-electron properties or efficacy required for standard MRI contrast enhancement.