The argument's flaw is about what causes comfort when approaching strangers specifically. (A) raises a different issue: whether you'd feel uncomfortable approaching someone only if that person is a stranger. That's about the stranger/non-stranger distinction, not about what makes you comfortable with a stranger in particular. The argument never assumes anything about how you'd feel approaching non-strangers, so this isn't a flaw it commits.

The author isn't reasoning from similar situations. One of her premises directly states that most long-term friendships begin because someone felt comfortable approaching a stranger. She's not reasoning from cases similar to long-term friendships; she's working directly with a claim about most long-term friendships.

Like (A), this focuses on the stranger/non-stranger distinction. (C) is about whether you're less comfortable approaching a same-age person who's a stranger versus a same-age person who isn't a stranger. But the argument doesn't assume anything about how comfort levels compare between strangers and non-strangers. The flaw is about what leads to comfort when approaching strangers, not about whether stranger status itself affects comfort.

(D) doesn't matter here, because the second premise already narrows our focus. We're told that most long-term friendships begin because someone felt comfortable approaching a stranger. So even if people sometimes approach strangers while uncomfortable, the long-term friendships we care about are specifically the ones that started with comfort.

This accurately describes what the author overlooks. The argument tells us that being the same approximate age makes you likely to feel comfortable approaching a stranger. But it never addresses whether you'd also feel comfortable approaching a stranger who isn't your approximate age. If you would, then "felt comfortable approaching a stranger" doesn't reliably indicate "same approximate age." Those long-term friendships might involve differently-aged people who shared the same hobby or who met at work, etc., because there could be other paths toward feeling comfortable approaching a stranger besides being of the same age.