The correct option is D NO CHANGE
When two clauses are combined together, and one of them is dependent on the other for its full meaning, then that clause is subordinate (they cooked a pavlova with them) to the main clause (The chefs had only fruit and eggs in the pantry). Hence to combine these, we use a subordinating conjunction. The subordinate clause is the result or effect of the main clause. The chefs had only fruit and eggs, the result was that they had to cook a pavlova with them. 'So' is the subordinating conjunction which denotes the result of the main clause, hence the underlined word does not need to be changed, and option D is correct. 'After' is a subordinating conjunction that refers to time-related sentences, since the given sentences are not time-related, option A is incorrect. 'But' is a coordinating, adversative conjunction used for sentences that are independent before they're combined, and they're both contrast in meaning. Since one of the given sentences is not independent nor contrast in meaning, option B is incorrect. 'As' is a subordinating conjunction used when the subordinate clause acts as a cause or reason, since the given subordinate clause here is not the cause, but the effect, option C is incorrect.