Option a may be a tower. Tower means to achieve great heights. However, it's not an appropriate word to be used for prices.
Option b is slow. Being slow refers to hurry. However, the context of cash isn't requiring any relevancy speed.
Option c is fall. after we use the word ‘fall’ within the given sentence, it doesn't convey the correct meaning. How can the costs of products be less when the merchandise available is a smaller amount in quantity?
Option d is soar. Soar means to rise very high. However, it also means to rise very quickly above a specific level. If we use this word within the given sentence, we will see that it's appropriately conveying the meaning of the full sentence, that is, the costs of products which are short in supply rises very rapidly above a particular level.Thus, from the reason of the given options, we will see that the proper option is option d because using it within the sentence conveys the meaning of the sentence perfectly.