Step 1: Establishment of AITUC.
The All India Trade Union Congress (AITUC) was founded in 1920. Lala Lajpat Rai was its first president.
Step 2: Analysis of options.
(A) Bal Gangadhar Tilak: Died in 1920 before AITUC formation.
(B) Lala Lajpat Rai: Correct — first President of AITUC.
(C) Gopal Krishna Gokhale: Died in 1915.
(D) Subhash Chandra Bose: Later associated with labor movements, but not the first president.
Step 3: Conclusion.
The first President of AITUC was Lala Lajpat Rai.