What is the definition for Ackermann’s function?

(a) A(1,i) = i+1 for i>=1

(b) A(i,j) = i+j for i>=j

(c) A(i,j) = i+j for i = j

(d) A(1,i) = i+1 for i<1

The query is from Trees in section Trees of Data Structures & Algorithms I

This question was addressed to me during an online interview.