The correct answer is: c) 1-pentanol.
In mass spectrometry, the M+1 peak corresponds to the molecular ion peak of the compound plus one mass unit, which is typically due to the presence of isotopes of elements like carbon and hydrogen in the molecule. 1-pentanol (C5H12O) has a significant number of carbon atoms, which increases the likelihood of detecting the M+1 peak. Each carbon atom has a natural abundance of carbon-13 isotope, which contributes to this peak.
In comparison, hexane, 1-chloropentane, and 3-methylpentane may produce M+1 peaks as well, but the intensity would not be as prominent as that of 1-pentanol due to the presence of the hydroxyl group in 1-pentanol, which also enhances the stability and detectability of the molecular ion.