The assertion is TRUE because when a selection medium containing Ampicillin is sterilised by autoclaving, the heat destroys the antibiotic. Since Ampicillin is a heat-labile substance, autoclaving renders it ineffective. As a result, there is no functional ampicillin left in the medium to inhibit $amp^{S}$ (ampicillin-sensitive) microbes, so both $amp^{S}$ and $amp^{R}$ microbes show growth. The reason is also TRUE and correctly explains the assertion. According to the textbook context, 'Filtration is used to sterilise the heat labile substances in solutions, such as serum, antibiotics, sugars, urea, etc.' Since ampicillin is an antibiotic and heat-labile, it should be sterilised by filtration using membrane filters (0.2–0.45 µm pore size) before adding to the medium, not by autoclaving. The reason correctly explains why both microbes grew - because the wrong sterilisation method (autoclaving) was used, which destroyed the ampicillin.