I think it's equally helpful, if u can break the sentence part by part...
Main sentence
Although appearing less appetizing than most of their round and red supermarket cousins, heirloom tomatoes, grown from seeds saved during the previous year – they are often green and striped, or have plenty of bumps and bruises-heirlooms are more flavorful and thus in increasing demand
Now, let's see who is modifying what..
1. "grown from seeds" > heirloom tomatoes
2. "saved during the previous year" > seeds
3. "— they are often green and striped" > heirloom tomatoes (here dash (-) is used instead of comma)
4. "— heirloom are more flavorful and thus in increasing demand." (here dash (-) is used instead of semicolon)
Now, for our simplicity, let's give a short name for each long clauses.
1. most of their round and red supermarket cousins > X
2. often green and striped, or have plenty of bumps and bruises > "actually Y"
3. are more flavorful and thus increasing in demand. > "do Z"
Eliminate the modifier "grown from seeds saved during the previous year" which is unnecessary
Now rephrase each answer choices with including nonunderlined portion of main sentence, try to find out the inconsistencies of each sentence.
1. Although appearing less appeatizing than X,
Heirloom tomatoes they are actually Y; Heirlooms do Z. (Redundant)
2. Although heirloom tomatoes appear less appetizing than X, they are actually Y; Heirlooms do Z.
3. Although they appear less appetizing than X, Heiloom tomatoes,
they are actually Y; Heirlooms do Z. (Redundant)
4. Heirloom tomatoes appear less appetizing than X, they are actually Y; Heirlooms do Z (conjunction missing; two independent clauses cannot be connected without conjunction or semicolon or dash)
5. Heirloom tomatoes, although they appear less appetizing than X,
they are actually Y; Heilooms do Z. (Redundant)
Hope, it helps a lot.