The answer is 3 input XOR gate.
M (a,b,c)=ab+bc+ca
M (M (a,b,c)',M (a,b,c'),c)
Consider
M (a,b,c)'=x
M (a,b,c')=y
To find M (x,y,c)
= xy+yc+cx (from given def of M)
Now substitute the values pf x and y
=(ab+bc+ca)'(ab+bc'+c'a) + (ab+bc'+c'a)c + c (ab+bc+ca)'
You need to expand the above exp and then reduce it accordingly.
In the end you will get
C XOR a XOR b