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a. You wouldn't use an isotope with a half-life similar to that of carbon-14 to determine the age of the Solar System because the timescale involved in the formation of the Solar System is much longer than the half-life of carbon-14. Carbon-14 is commonly used for dating relatively recent materials, up to around 50,000 years, due to its relatively short half-life. However, the age of the Solar System is estimated to be around 4.6 billion years. Using an isotope with a shorter half-life would limit the dating range to much shorter timescales, making it unsuitable for determining the age of the Solar System.
b. No, the age of the universe being approximately 14 billion years does not mean that no rubidium-87 has decayed yet. Rubidium-87 has a very long half-life of 49 billion years, which means that it decays extremely slowly. Even though the age of the universe is estimated to be around 14 billion years, there has still been ample time for some rubidium-87 to decay. However, due to its long half-life, the decay of rubidium-87 is extremely gradual, and the amount of decayed rubidium-87 relative to the total amount is relatively small compared to isotopes with shorter half-lives. Therefore, while some rubidium-87 may have decayed since the formation of the universe, the majority of it would still remain undecayed.