When Mendel's first-generation pea plants (Hh) were allowed to self-pollinate (Hh x Hh), the expected ratio of tall (dominant) to short (recessive) plants in the second generation is 3:1. This is due to the following Punnett square results:
- HH (homozygous dominant) - Tall
- Hh (heterozygous) - Tall
- Hh (heterozygous) - Tall
- hh (homozygous recessive) - Short
Thus, there are 3 tall plants to every 1 short plant, leading to the ratio of 3:1.
The correct answer is D: 3:1.