Your question concerns pea plants. In peas, the gene for "tall" (T) is dominant, and the gene for "dwarf", or short (t) is recessive. When a plant that is homozygous dominant for "tall" is crossed with one that is homozygous recessive for "short", each parent can only give one type of gene. The tall parent is referred to as having a genotype of TT, and can only give "T" alleles; the short parent is of the tt genotype, and can only give "t" alleles. Each offspring in the F1 generation is of genotype Tt; their phenotype (appearance) is that they are all tall because the gene for tall is dominant. This is your answer for part a of your question.
When the F1 plants (all Tt) are crossed with each other, however, each parent has an equal chance of passing on either the gene for tall (T), or the gene for short (t). If this information is used in a Punnett square, the F2 generation will have 25% of the plants TT (phenotype tall) 50% Tt (also tall, due to the dominant tall gene), and 25% tt (phenotype short), as only the plants with two recessive genes will show the trait for being short. Therefore, 75% of the F2 generation will be tall and 25% short; the ratio is 3:1. This is your answer for part b.
For part c of your question: the F2 generation is different from the F1 because the parent plants of each are different. The original parent generation could only give one type of gene each; the F1 generation, when they are crossed, could each give either type.