Answer:
d) 'Whoever travels the Pelly river' follows in the footsteps of the explorer Robert Campbell.
Explanation:
In a sentence that contains both a dependent and an independent clause, the clause can either be presented/ function as a subject clause or object clause. A noun clause that can function as a subject clause will be best identified by the use of the word "who, whatever, that, where, wherever" etc among many other words that suggests the function. And since noun clauses are only dependent clauses, meaning, they cannot be taken as a full fledged sentence, they need an independent clause to give the full meaning of the whole sentence.
Among the given sentences, the only noun clause that functions as a subject is option (d). The other options are all instances where the noun clause is used as the object while option (d)'s noun clause is used as a subject. It starts the sentence, and also behaves as the main subject of the whole sentence.
