First blank: insidious - proceeding in a gradual, subtle way, but with harmful effects, it means the slow permeation of technology. If one found if ominous, then it wouldn't be correct to state it as beneficial as well.
Second blank: tolerable - the statement agrees that can technology can be tolerable to the extend to which it can even become beneficial. The positive idea conveyed increases, so a less positive word to describe this blank.
Third blank: alienation - something that is negative in connotation. Even though technological spread can be beneficial, it can have negative effects like alienation. It ties up well with insidious

The answer for the (ii) blank should be D

The correct words for the blanks are (i) insidious, (ii) salutary, and (iii) alienating.

Breakdown of the Blanks
- Blank (i): insidious

The sentence states that critics are "most alarmed" by the nature of technology. The second half of the sentence clarifies this concern: they worry about its "ubiquity" and that it can be "alienating." This implies a hidden or subtle negative quality. Insidious means proceeding in a gradual, subtle way, but with harmful effects. This word perfectly captures the critics' fear that technology's widespread presence is subtly harming culture.
- Blank (ii): salutary

The word "Though" sets up a contrast. The critics have a main concern (insidious nature), but they also acknowledge some positive features. Salutary means producing good effects; beneficial. This fits the context of them finding some of technology's features positive and even beneficial.
- Blank (iii): alienating

The final blank describes the negative effect of technology's "ubiquity." Alienating means causing someone to feel isolated or estranged. The critics' worry is that technology's omnipresence makes people feel disconnected from each other or their environment. This fits the description of a harmful social effect.

1) Is it salutary or tolerable ?
2) Ominous also means a feeling that something bad is going to happen in the future, right ? Why is it wrong ?

The best answers for each blank, based on context and meaning, are:
- Blank (i): insidious
- Blank (ii): salutary
- Blank (iii): alienating

Here's the sentence filled in:

It is the insidious nature of technology that most alarms cultural critics. Though they find many of its features salutary and even beneficial, they worry that its ubiquity can be alienating.

Explanation
- Insidious means something harmful in a subtle or gradual way, which fits well for the way critics might perceive technology's pervasive, sometimes hidden, negative effects.
- Salutary means beneficial or promoting good results, fitting for features critics consider positive.
- Alienating means causing someone to feel isolated or estranged, which cultural critics often warn can be an effect of overwhelming technology.