In the above mentioned three sentences, the most appropriate sentence to be in the second position is Option A.
The first sentence describes about a situation where boiling an egg is considered a simple job and the last sentence speaks about how boiling an egg is too difficult at such high altitude. Thus, the availability of the water is what fits into the second position (option A), given the nature of all the three sentences.
The other options B, C and D speak about situations which are nowhere related to the sentences given and which do not fit in the second position given the first and the last sentence.