Step 1: Identify the fixed expression.
The sentence contains the phrase
\[
\boxed{\text{in accordance with}}
\]
which is a commonly used English expression.
Step 2: Recall its meaning.
The expression in accordance with means
• in agreement with,
• according to, or
• following the prescribed rules or guidelines.
Since the proposal followed the guidelines issued by the headquarters, the correct preposition is in.
Step 3: Analyse the options.
• (A) under -- Incorrect. "Under accordance" is not a correct phrase.
• (B) in -- Correct. It forms the correct expression "in accordance with".
• (C) on -- Incorrect. "On accordance" is grammatically incorrect.
• (D) with -- Incorrect. "With accordance" is not used in English.
Step 4: Final conclusion.
\[
\boxed{\text{The proposal was in accordance with the guidelines.}}
\]
Hence, the correct option is \(\boxed{(B)}\).