SELECT 
  variant_id, 
  modifier, 
  nav_skus 
FROM 
  cscart_product_option_variants 
WHERE 
  option_id = 23524 
  AND nav_skus = '100'

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_option_variants",
      "access_type": "ref",
      "possible_keys": ["option_id", "option_id_2"],
      "key": "option_id",
      "key_length": "3",
      "used_key_parts": ["option_id"],
      "ref": ["const"],
      "rows": 39,
      "filtered": 100,
      "attached_condition": "cscart_product_option_variants.nav_skus = '100'"
    }
  }
}

Result