Select the option that can be used as a one-word substitute for the given group of words.
A doubtful look or examination of one's motives.

The correct answer is Suspect.

Let us analyze the meaning of the given phrase and the provided options to find the correct one-word substitute:
"A doubtful look or examination of one's motives" relates to feeling doubtful, distrustful, or questioning the truth or honesty of something or someone.

Let's look at the definitions of all the options:
1. Introspect: To examine one's own thoughts, feelings, or mental processes. This is a self-examination but is reflective rather than "doubtful" in nature.
2. Suspect: To have a doubt or feeling that something is true, or to look at someone or something with distrust, doubt, or suspicion. This fits the description of a doubtful look or examination of motives.
3. Circumspect: Wary and unwilling to take risks; cautious.
4. Retrospect: A survey or review of a past course of events or period of time (looking back).

Therefore, Suspect is the most appropriate one-word substitute for the given group of words.