JSFiddle - React, Tailwind, and code Playground

HTML

<select id="select1"></select>

JavaScript

$(document).ready(function() {
  var jsonstirn = '[{"capacity":"4Seater","carname":"car1","price":"83"},{"capacity":"4Seater","carname":"i10","price":"83"},{"capacity":"4Seater","carname":"i20","price":"83"},{"capacity":"8Seater","carname":"car3","price":"83"}]';
  var jsonarray = JSON.parse(jsonstirn);
  var capacity_val = '';

  //Set will hold unique set of elements.
  var uniqueCapacity = [...new Set(jsonarray.map(v => v.capacity))]
    .map(v => '<option value="' + v + '">' + v + '</option>').join("");
  //Map the unique set of elements with required string values.

  $('#select1').html(uniqueCapacity);
});