This is an UP counter, counting starts from 000( as counter is initially reset) and increments by 1 on each pulse.
Q3Q2Q1Q0
0 0 0 0
when the outputs of Q2 and Q3 are 1 then NAND gate will produce o/p as 0 and it will clear all the flip flops.
Hence, answer will be mod 12 machine (0-11).