～さ

この果物（くだもの）の_は食（た）べてみないとわからない。
a. おいしい b. おいしく c. おいしさ d. おいしみ

The intended meaning is: People cannot get to know how delicious this fruit is unless they taste it. The topic marker は follows the blank, so the word in question needs to be a noun, and the correct answer here is c. おいしさ.

Most adjectives can be changed to a noun by changing the ending to さ (in case of いadjectives) or adding さ (in case of なadjectives).

暑（あつ）さ寒（さむ）さも彼岸（ひがん）まで
Higan (the equinoctial week) marks the end of both the worst of summer and the worst of winter.

必要（ひつよう）な大（おお）きさに切（き）ります。
I will cut it to the size you require.

私（わたし）は心（こころ）の狭（せま）さを反省（はんせい）しています。
I regret my narrow-mindedness.

太平洋（たいへいよう）の深（ふか）さはエベレスト山（ざん）の高（たか）さを超（こ）えている。
The depth of the Pacific Ocean exceeds the height of Mt. Everest.

悲（かな）しさのあまり、眠（ねむ）ることができない。
I cannot sleep in the excess of my grief.

彼（かれ）の真面目（まじめ）さは折（お）り紙（がみ）付（つ）きだ。
We can guarantee he is very serious.

子どもたちにやってみることの大切（たいせつ）さを教（おし）えたい。
I want to teach children the importance of trying.

彼女（かのじょ）の丁寧（ていねい）さは半端（はんぱ）じゃない。
Her conscientiousness is incredible.

You can also use み with some adjectives to make them into a noun, but the meaning of ～さ and ～み are not the same. ～さ can usually be quantified while ～み is what is described by the adjective itself.

川（かわ）の深（ふか）さ = the depth of the river
川（かわ）の深（ふか）み = a deep spot in the river

この薬（くすり）の苦（にが）さはあの薬（くすり）の二倍（にばい）だ。
The bitterness of this medicine is twice as much as that of that medicine. (=This medicine is twice as bitter as that medicine.)
私はコーヒーの苦（にが）みが好（す）きだ。
I like the bitterness of coffee.

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