Which feature of four different marine fossils makes the fossil organism most similar to the present-day organism with four limbs?

I found the rest of the exercise on the internet and these are the options.
"-Undifferentiated vertebrae throughout the body.
-One neck vertebra allowing up and down movement.
-Many neck vertebrae that allow for greater mobility.
-Two neck vertebrae allowing up and down and sideways movement."

The correct answer would be the third. - "Many neck vertebrae that allow for greater mobility." The present-day organism with four limbs has a neck with several vertebrae that allow movements of rotation (mostly because of only two vertebrae at the top of the neck), and side way movements, as well as back and forth movements.

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.