Question $1$: Measure over Trees $(24$ pts$)$

The purpose of this question is to get you to explore measure$-$theoretic probability more deeply. To

that end, we are going to ask you to construct a probability space for a probabilistic context$-$free

grammar $($PCFG$).$ Consider the following context$-$free grammar $($CFG$)$

$SSS$

$Sa$

where $S$ is the only nonterminal and $a$ is the only terminal.

I am confused with question c$,$ please only answer question c$,$ and please give a comprehensive answer for C$.$