Only some strains of the tobacco plant are naturally resistant to toba

Dear Not Random,
I'm happy to help. I don't know the source of this question, but it is a very high quality question, and reflects well the priorities of scientific research. You see, at its core, science is a very conservative undertaking, and given a bit of evidence, scientists are generally willing to offer only the most limited of conclusions.

Some things to consider about this scenario
(1) We know the resistant plants make a ton of salicylic acid, but we don't know that salicylic acid is absent in the non-resistant plants --- just that they don't have a big increase of it when they get infected.
(2) We don't know whether the plants that were able to resist the virus resisted purely because of salicylic acid, or because of some synergistic combination of salicylic acid and some other factor. Maybe the salicylic acid stimulates the plant to secrete something else, and this something else fights the virus. There are many possibilities to consider.

(A) Tobacco plants that have become diseased by infection with tobacco mosaic virus can be cured by injecting them with salicylic acid.
This entire argument is about prevention --- preventing the tobacco plants from getting the disease. There are all kinds of things (e.g. inoculations) that work wonderful as prevention before someone has the disease, but do absolutely nothing to cure the disease in someone who already has it.

(B) Producing salicylic acid is at least part of the mechanism by which some tobacco plants naturally resist the disease caused by tobacco mosaic virus.
This is very conservative, in the manner of the scientific method. The evidence suggests that the salicylic acid must plan some role. This is promising.

(C) Salicylic acid is not produced in strains of tobacco plants that are not resistant to tobacco mosaic virus.
This is the error discussed in #1 above.

(D) It is possible to test an uninfected tobacco plant for resistance to tobacco mosaic virus by measuring the level of salicylic acid it contains.
Hmmm. Is it the level of salicylic acid, or the rate of increase in salicylic acid that makes a difference? This is not clear from the passage, so this choice is not a clear conclusion.

(E) The production of salicylic acid in certain strains of tobacco plants can be increased and thus the strains made resistant to tobacco mosaic virus.
Hmm. That first part is really iffy ---- could we genetically engineer the tobacco plants so that they produce more salicylic acid? Maybe, or maybe not. Genetic engineering can certainly do a whole lot, but could it do this? Unclear. Therefore, we don't know whether the production of salicylic acid could be increased by human intervention.

The best answer, really the only viable answer, is (B).

Does all this make sense?
Mike