Possible answers:
(I). The stalk will be visibly tender to the touch. You may also notice scales that flake off when you check for them.
(II). In general, full mature romaine lettuce will have a wavy, wilted appearance that resembles wilted cabbage. Purchase romaine lettuce from a farmer's market.
(III). Once they've progressed enough for you to get a sense of their bright green color, it's mature. If you spend your time gardening, you can stop by a farmer's market to buy romaine lettuce.
(IV). Unlike iceberg lettuce, the romaine heads will not be densely closed in upon themselves when mature. Cut the entire head off above the base if you want a second harvest.Choose from options above and answer: What most naturally follows?

How to harvest romaine lettuce 
Harvest the romaine heads about 65-70 days after planting the seeds. 
When grown from seeds, romaine lettuce takes only a little less than 3 months to fully mature. You can tell when the heads are mature by their visual appearance: they'll have a dark green color and look leafy and open.
Answer:
(IV).