From the given alternative words, select the word which cannot be formed using the letters of the given word.
ILLUSTRIOUS

Given: ILLUSTRIOUS

Letters in "TRUST": T, R, U, S, T
The word "ILLUSTRIOUS" contains only one 'T', but "TRUST" requires two 'T's. So, "TRUST" can not be formed.

For  RUST;
Letters in "RUST": R, U, S, T
Letters in "ILLUSTRIOUS": R, U, S, T are all present. So, "RUST" can be formed.

For  LUST;
Letters in "LUST": L, U, S, T
Letters in "ILLUSTRIOUS": L, U, S, T are all present. So, "LUST" can be formed.

For RIOT;
Letters in "RIOT": R, I, O, T
Letters in "ILLUSTRIOUS": R, I, O, T are all present. So, "RIOT" can be formed.

So, "TRUST" can not be formed using the letters of the given word.

Thus, the correct answer is (a).