Q.What is common to Rajya Sabha and Lok Sabha election methods: 18+ voter eligibility / preference-order voting / equal vote value / >50% winner requirement?

Lok Sabha MPs are elected directly by all adult voters under FPTP (one mark, most votes wins); Rajya Sabha MPs are elected indirectly by MLAs using preference ranking under STV. Despite this difference in method, one thing is shared.

Checking each option:

• Every citizen above 18 is an eligible voter: true for the Lok Sabha, since all adult citizens vote directly. Not true for the Rajya Sabha — its "voters" are the MLAs of each State legislature, not ordinary citizens, so this is not common to both.
• Voter can give preference order for different candidates: true for the Rajya Sabha, which uses the Single Transferable Vote system with ranked preferences. Not true for the Lok Sabha, where under FPTP a voter simply marks one candidate — no ranking at all. Not common to both.
• Every vote has equal value: in the Lok Sabha, every citizen's vote counts as one, with no vote worth more than another. In Rajya Sabha elections, every MLA's vote also counts as one (unlike, say, Presidential elections, where votes are deliberately weighted by population/seats) — so this genuinely holds in both systems.
• The winner must get more than half the votes: not true for the Lok Sabha, where FPTP only requires the most votes, not a majority. Not exactly true for the Rajya Sabha either, where the winning threshold is a calculated quota, not simply "over 50%." Not common to both.