To solve this problem, we need to select the third (R), fifth (H), sixth (A), and eleventh (E) letters from the word MERCHANDISE, and then form words using each letter only once.

The selected letters are R, H, A, and E.

Now, let's form words using these letters:

1. HEAR
2. HARE

Both words are meaningful and fulfill the conditions given in the question.

Now, we need to find the third letter of these words:

1. HEAR - The third letter is E.
2. HARE - The third letter is R.

As there are two possible words with different third letters, the answer is 'T'. So, the correct option is (E) T.