Introduction:
The given problem requires us to determine the likely starting letter of a poem that follows certain conditions. We are given seven lines, each starting with a different letter (P, Q, R, S, T, U, V), and we need to find the most probable starting letter of the poem.

Given Conditions:
1. The line starting with R is the third from the bottom.
2. The line starting with V is the second from the top.
3. The line starting with Q and the line starting with U have exactly three lines in between.
4. Either S or P is the only letter between R and U.

Solution:
To solve this problem, let's analyze the given conditions and determine the possible starting letters.

Condition 1: The line starting with R is the third from the bottom.

This condition tells us that the line starting with R is not the first or second from the top. Therefore, it cannot be the starting line of the poem. Thus, we can eliminate R as a possible starting letter.

Condition 2: The line starting with V is the second from the top.

This condition tells us that the line starting with V is not the first from the top. Therefore, it cannot be the starting line of the poem. Thus, we can eliminate V as a possible starting letter.

Condition 3: The line starting with Q and the line starting with U have exactly three lines in between.

This condition tells us that the line starting with Q and the line starting with U have three lines between them. To satisfy this condition, the possible arrangement could be:

Q _ _ _ U

Condition 4: Either S or P is the only letter between R and U.

This condition tells us that either S or P is between R and U. Therefore, the possible arrangements could be:

R S U
or
R P U

Conclusion:
By considering all the given conditions, we can conclude that the poem is likely to start with either T, S, or P. Therefore, the correct answer is option 'C': Only T, only S, or only P.