At each stage of production, sterility of the preparation is a major consideration. when considering appropriate measures to implement to control microbial growth, one must think about the susceptibility of possible microbial contaminants. apply your knowledge of the structural features of microbes and the mechanisms of action for antimicrobial agents to rank the following microbes according to their susceptibility.

gram-negative bacteria
prions
endospores
enveloped virus
grampositive bacteria

According to the susceptibility towards antimicrobial agent the ranking from most susceptible to least susceptible is as:  

enveloped virus > gram-positive bacteria > gram-negative bacteria > endospores > prions

Enveloped viruses are most susceptible because their envelope is made up of lipid which is easily disrupted or degraded by antimicrobial agents like detergent.

Gram-positive bacteria are only having one layer of cells wall that is made up of peptidoglycan which is susceptible to most antibiotics but gram-negative bacterial have an extra outer layer of covering made up of lipopolysaccharide so many of them show multidrug-resistance.

Endospore are among the most resistant microbial structure which is formed outer to cell wall and do not allow most antimicrobials to enter the cell.

Prions are infectious protein so antimicrobial agent do not work on prions and it also resist its chemical degradation therefore it is least susceptible.  

b would be the right answer