Which of the following describes something that is harmful in effect and hostile?

The term "inimical" is defined as being harmful or hostile, making it the correct description for something that poses a negative effect on others. It encompasses both the notion of being damaging or unfavorable and the characteristic of being unfriendly or antagonistic. This dual connotation of harm and hostility makes "inimical" particularly apt in contexts where one is addressing behaviors, actions, or entities that actively work against the interests or well-being of others.

In contrast, the other options do not carry the same meaning. For example, "inexorable" refers to something that is relentless or unyielding, which does not inherently convey hostility or harm. "Insipid" describes something that is bland or lacking flavor or interest, which also does not imply hostility. "Indomitable" means unconquerable or impossible to subdue, which reflects resilience rather than an adverse effect. Hence, "inimical" stands out as the most appropriate term in this context.