Early Buddhist iconography often uses symbolic, aniconic forms instead of direct human images of the Buddha.

In that symbolic scheme, the empty seat or empty throne represents the Buddha's meditative presence. Therefore the accepted answer is the Buddha's meditation.

Statement 2 is the trap. Amaravati was not "next only to Sanchi" in size; it was larger in scale.

Amaravati Stupa is considered one of the largest and most elaborate Buddhist stupas of ancient India. Many historians believe it surpassed Sanchi Stupa in overall scale and artistic richness during its peak.

Sanchi Stupa is the largest surviving.

so statement 2 is false.

That leaves statements 1 and 3 as the valid combination.