Step 1: Understand what the Code did.
The Industrial Relations Code, 2020, is one of the new labour codes. Its job was to merge several older labour laws into one single, simpler law.
Step 2: Which laws it merged.
The Code brings together three older Acts: the Industrial Disputes Act, 1947; the Trade Unions Act, 1926; and the Industrial Employment (Standing Orders) Act, 1946. These three are all about the relationship between workers and employers.
Step 3: Look at the odd one out.
The Industries (Development and Regulation) Act, 1951 is about industrial licensing and controlling production capacity of industries. It is not about disputes, unions, or worker-employer relations.
Step 4: Match the theme.
The whole theme of this Code is industrial relations. The IDR Act, 1951 has a different theme - development and regulation of industries - so it simply does not fit inside this Code.
Step 5: Check the other options.
The Industrial Disputes Act, Trade Unions Act, and Standing Orders Act are all clearly part of the Code, so none of those can be the answer to a not included question.
Step 6: Conclude.
The one law that is not part of the Industrial Relations Code, 2020 is the IDR Act, 1951.
\[ \boxed{\text{Industries (Development and Regulation) Act, 1951 (IDR Act)}} \]