= Solution
{c}
With the rate $r=0$, the <risk-neutral probability in a binomial market> must satisfy
$$
q u_i+(1-q)d_i=1,\qquad i=1,2.
$$
The first stock gives $0.8+0.6q=1$, hence $q=1/3$. The second gives $0.9+0.3q=1$, confirming the same answer. Thus
$$
\boxed{(q,1-q)=(1/3,2/3).}
$$
Take the successive up/down choices independent under this measure, so both stock prices are <martingales>. The martingale transition condition applies from times zero to one and one to two; the displayed $t+1$ condition in the source should be read for these two transitions.
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