When mendel’s first generation pea plants were allowed to self-pollinate, the ratio of tall (dominant) to short (recessive) plants in the second generation was a) 1: 1. b) 1: 3. c) 2: 2. d) 3: 1.

The ratio of tall to short in the second or the F2 generation was 3:1. Because the resulting genotype of the F1 generation would be Pp. If you test cross that and put in into a Punnet square, you would have three blocks that have the dominant trait and one block without. Remember that the dominant trait is always expressed. So three of them would be tall and one would be short. 

You answer is then D.