The correct option is
D He saves some money regularly so that he could use it in his old age.
The given two sentences are in different tenses. The first sentence is in the present tense (saves some money regularly). The second sentence talks about a probability in future (could use it in his old age). The first event in the past leads to the second event in the future i.e one is the cause for the other.
"So that" is a subordinating conjunction which is used to express cause or reason. "He saves some money regularly so that he could use it in his old age" means that the reason for his "saving regularly" in the present is in order to be able to "use it in his old age". Hence, option C is correct.
Option A: "As soon as" is a subordinating conjunction which is used to connect an action or an event to a point in time. It is used when one action/event immediately follows the other. In this case, the two events are in different times-one in the present and the other in the future. Hence, this option is incorrect.
Option B: "As well as" is a replacement for the correlative conjunction "not only... but also". It is a conjunction which is used to connect two clauses which belong to the same part of speech. If the word which precedes "as well as" is a verb, then the word which follows "as well as" should also be a verb. Here, "as well as" is preceded by an adverb (regularly) and followed by a pronoun (he). Hence, this option is incorrect.
Option D: "However" is a co-ordinating adverb which is used to connect two contrasting independent clauses. Here, the two clauses are not contrasting. Hence, this option is incorrect.