- when galaxies were first formed.
- when hydrogen and helium were formed.
- when gravity was quantum mechanical.
- the time of recombination.
1. It doesn't make sense for galaxies to be the oldest observable objects; galaxies are composed of gas, dust and stars, all of which interact with electromagnetic radiation that we can detect. At least some of these objects must have formed BEFORE its environment could be considered a galaxy.
2. This one's a bit more complicated to explain. Hydrogen and helium were formed in the early universe, but in order to observe them with a telescope, we would need to be able to observe some aspect of electromagnetic interaction with them. This would require that electromagnetism existed as we currently understand it, and that its radiation was interacting with hydrogen and helium in observable ways. Unfortunately, in the early universe, matter was effectively a particle soup, with protons, electrons and photons all interacting with each other, rendering this period opaque to EM radiation.
3. Quantum gravity is still largely hypothetical, and is nevertheless largely irrelevant to the operation of a telescope.
4. Recombination describes the period at which the early particle soup was able to condense into distinct atoms, and photons became free to travel. Thus, this is the first time in the history of the universe at which photons were capable of being observed by anything outside the original particle soup. 4 is the correct answer, although elements of 2 are correct as well, because hydrogen and helium would have formed at the time of recombination.