Step 1: Understanding the problem.
In a league tournament with 10 teams, each team plays against every other team exactly once. The number of matches can be calculated using the combination formula:
\[
\text{Number of matches} = \binom{n}{2} = \frac{n(n-1)}{2}
\]
where \(n\) is the number of teams. For \(n = 10\), we have:
\[
\text{Number of matches} = \frac{10(10-1)}{2} = \frac{10 \times 9}{2} = 45
\]
Step 2: Conclusion.
The correct answer is (B) 45, as 45 matches will be played in the tournament.
Final Answer: 45.