Select the best one word substitute for the informal expression You bet which means definitely or certainly.

Introduction / Context:
This question focuses on conversational English and one word substitution. The phrase You bet is a common informal reply in spoken English, especially in American usage. It usually appears when a speaker strongly agrees with a statement or happily confirms something. Examinations often test whether learners can match such informal expressions with more neutral, formal, or single word equivalents.

Given Data / Assumptions:

• The expression provided is You bet.
• Four options describe different actions or meanings.
• Only one option correctly captures the sense in which You bet is commonly used.
• The phrase is considered as a response in conversation, not in the context of gambling.

Concept / Approach:
In everyday conversation, if one person asks, Will you help me tomorrow, a friendly response might be You bet, which means Of course or Certainly. Although the word bet by itself relates to gambling, the fixed phrase You bet usually does not refer to actual betting. Instead, it functions as a strong, positive confirmation. Therefore, the correct one word substitute among the options is certainly. Options that mention contradiction, gambling as an activity, or letting others lead do not match the typical conversational usage of You bet.

Step-by-Step Solution:
Step 1: Recall where you have heard or seen the phrase You bet. It usually appears as a quick, enthusiastic yes. Step 2: Check option a, Contradicting someone. This describes disagreement, which is the opposite of the agreement conveyed by You bet. Step 3: Check option b, Certainly. This is a polite and strong yes, matching the tone and meaning of You bet. Step 4: Check option c, Gamble. While bet alone refers to gambling, the phrase You bet in conversation is not literally about placing a wager. Step 5: Check option d, Letting others lead. This indicates stepping back or allowing others to take charge, not confirming a statement. Step 6: Conclude that certainly is the best one word substitute for You bet in its conversational sense.

Verification / Alternative check:
Imagine some dialogues. Example one: Are you coming to the match You bet. If we replace You bet with Certainly, the dialogue remains natural and carries the same positive confirmation. Example two: Can I borrow your notes You bet. Again, certainly fits well. Now try replacing You bet with Gamble or Contradicting someone; those substitutions do not make sense in context. This mental check confirms that option b is correct.

Why Other Options Are Wrong:
Contradicting someone: The phrase You bet does not signal disagreement; it signals strong agreement or assurance.
Gamble: While related to betting in a literal sense, the conversational phrase You bet usually has nothing to do with actual gambling.
Letting others lead: This is unrelated to answering a question in the affirmative and does not express confirmation.

Common Pitfalls:
A common trap is to focus only on the literal meaning of the word bet and assume that the entire phrase must refer to gambling. Another pitfall is ignoring how phrases function in real dialogues. Learners should pay attention to English subtitles, audio clips, and reading materials to familiarise themselves with idiomatic expressions and their real usage. This helps avoid confusion between literal and idiomatic meanings.

Final Answer:
The expression You bet most closely means Certainly, so option b is correct.