Step 1: Spot the two laws here.
A is 18 years old, of sound mind, and has signed a promissory note. Two laws matter: the Indian Majority Act, 1875 (who is an adult) and the Negotiable Instruments Act, 1881 (rules for a promissory note).
Step 2: Is A old enough to be bound.
Under the Indian Majority Act, a person becomes a major at 18. So an 18-year-old of sound mind is fully competent to enter a contract. A is not a minor, so the note is not voidable for age.
Step 3: Does the note need to state consideration.
A promissory note does not have to mention consideration on its face. The law already presumes that a negotiable instrument was made for consideration. So the note is not void just because the reason for payment is not written.
Step 4: Is a witness needed.
A promissory note between the maker and the payee does not need a third-party witness to be valid. The maker's own signature on a clear promise to pay a fixed sum is enough.
Step 5: Eliminate the wrong options.
Voidable because A is 18 - wrong, 18 is an adult. Void for not mentioning consideration - wrong, consideration is presumed. Valid only with a witness - wrong, no witness is required.
Step 6: Reach the answer.
The note has all the essentials, made by a competent adult, so A must pay B.
\[ \boxed{\text{The promissory note is valid; A is liable to pay B.}} \]