Most importantly, if you're picking an answer on Strengthen or Weaken that starts with "some", you better have verified that ALL the answers are worthless.

Some = at least one
It has almost no significant impact on any discussion. It's only if you're fighting a universal claim that "some" would have any impact.
If I say "All cakes are delicious",
you can weaken that by saying "some cakes are called urinal cakes". (gross, sorry)
But if I say "the great majority of cakes are delicious",
then it has no weakening effect for you to say "some cakes are made from worms and dirt".
I never said ALL, so your pointing out a putative exception is perfectly compatible with my original claim.
So on this problem, you could get rid of (D) for a few reasons:
1. the author said "a great majority of people trying to be President have been non-business. Few business execs seek to become president". So the quantity of "some" couldn't possibly hurt those claims.
2. The correct answer says "generally", which means "most", in more than half of cases. So a different answer is available with more punching power.
3. The argument is about whether business execs would be comfortable being President, not whether former Presidents would be comfortable later going into business (the professional life of a former-President is meaningfully different from that of the average person, so we can't really generalize anything from this).
4. The argument is about whether Prominent /
successful business
executives would be comfy trying to be President. When (D) talks about these former presidents, it just says they "engage in business ventures". That could mean they shovel your driveway or mow your lawn for $20. We have no reason to think that any former President has ever been a
successful or prominent business
executive.
hope this helps