Step 1: Read the facts.
A, an 18-year-old of sound mind, signs a written note promising to pay B 75,000 on 1st April next year. The note has all the features of a promissory note. A defaults and B sues. We must find the correct legal position.
Step 2: Know the law.
A promissory note is defined under Section 4 of the Negotiable Instruments Act, 1881. To be valid, the maker must have the capacity to contract and the note must have all its essential features.
Step 3: Check A's capacity.
A is 18 years old, so he has reached the age of majority. He is also of sound mind. So A is fully competent to make a promissory note.
Step 4: Check the essentials of the note.
The note has a written promise, an unconditional undertaking to pay, a fixed sum of money, the signature of the maker, and a certain payee, B. So all the legal requirements are met.
Step 5: Check the wrong options.
Option (B) is wrong because A is already a major, so the note is not voidable at his option. Option (C) is wrong because a witness is not needed for a promissory note. Option (D) is wrong because consideration is presumed under negotiable instruments law and need not be written out.
Step 6: Conclude.
Since A is a competent major and the note is complete, the promissory note is valid and A is liable to pay B.
\[ \boxed{\text{The promissory note is valid; A is liable to pay B.}} \]