What dose it mean for an allele to be dominant or recessive

a dominant allele of traits would be most likely to appear and be expressed by the individual, meanwhile a recessive allele of traits would be most likely to be “hidden” or wouldn’t be obviously expressed from an individual.

Explanation:

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?

if you listed your answers right then i believe your answer is b.