This question will test your comprehension, rather than your reasoning.
(A) The size of the smallest host system that a hacker could theoretically infect can be determined from the hacker’s virus implanting capability.'Determined' is not addressed to the 'Size' of hard disk, but to 'the chance of hacker to succeed in implanting the virus'. To determine if the hacker will succeed to infect the hard disk or not depends on her virus implanting capability.
(B) Host systems lack any effective defenses against the form of predation practiced by Online hackers.Out of Scope
(C) Online Hackers learn from experience how many viruses to implant into the harddrive of different host systems.Never mentioned where they learn the skills
(D) Failure to implant enough viruses would lead to the death of the developing viral streams more quickly than would implanting too many viruses.Trap- the argument does mention the importance of implanting enough virus but does not compare the effects of two different cases (i.e. too much/too less)
(E) Online Hackers use visual clues to calculate the size of a host harddriveAgain - Out of scope
For questions like these, one would eliminate the option A in the first read, but once one realizes that none other makes perfect sense, one can re-visit the A to see if the option is properly undertood. In case, you don't have much time, just mark anything that makes sence to you and move on.
This one is a time stealler.