What is the return value of the function foo when it is called as foo(513, 2)?

The function foo(n, r) computes the sum of the digits of $n$ represented in base $r$. When $r = 2$, it computes the sum of the bits (i.e., the number of set bits / $1$s) in the binary representation of $n$.

Convert $513$ to binary:

The number of set bits ($1$s) in the binary representation is $1 + 1 = 2$.

Hence, foo(513, 2) returns 2, which corresponds to option (D).