The answer to this question would be: d. replacing saturated fatty acids with unsaturated fatty acids.
To keep the membrane liquid, the bacteria must change the fatty acid into other structure with lower melting points. Saturated fatty acid has more C-C bind, make them less kinky and be able to be packed more tightly. This structure causes them to have higher melting and boiling point. So it was more likely to be solid than liquid if compared to unsaturated fatty acid.