The correct answer is: over.
The London Eye is a giant Ferris wheel on the South Bank of the River Thames in London. The nearest bridge to the London Eye is the Westminster Bridge, which crosses the Thames River. The Westminster Bridge is an over bridge, which means that it goes over the river.
The other options are incorrect. The London Eye is not above the Thames River, it is on the South Bank of the river. The London Eye is not off the Thames River, it is on the South Bank of the river. The London Eye is not towards the Thames River, it is on the South Bank of the river.