Why is this the answer?

The answer is suits because this is a very clever riddle that uses a secret code to describe the four different groups of symbols you find on a regular deck of playing cards. These groups are called the suits, and there are exactly four of them: Clubs, Diamonds, Hearts, and Spades. The riddle starts by saying, "Four of us are in your field, But our differences keep us at yield," which means these four different shapes are all used together on the card table to play a game and earn a "yield" or win. Let's break down the clues for each of the four suits. The first clue is, "First, a one that is no fool, Though he resembles a gardener’s tool." This is the Clubs suit. The Clubs symbol looks a little bit like a strong stick called a club, and the riddle is using a smart clue to point right to the name Club. The riddle continues, "Next, one difficult to split in two, And a girl once had one as big as her shoe." This is the Diamonds suit. Real diamonds are the hardest material on Earth, making them very difficult to "split in two." The diamond shape on the card looks like a beautiful, shiny jewel. Then we learn about the next suit: "Then, to the mind, one’s a lovely bonder, And truancy makes it grow fonder." This is the Hearts suit. The word "bonder" is a tricky way to say something that brings people together, and a heart is the symbol for love and friendship. The phrase "truancy makes it grow fonder" is a funny twist on the famous saying, "Absence makes the heart grow fonder," which means you miss things you love when you are away from them. Finally, the last clue is, "Last, a stem connecting dots of three." This points to the Spades suit. If you look carefully at the Spade symbol, it is a big, pointed shape that has a small, straight line or "stem" at the very bottom. This stem connects the curved parts of the design, which look like a single shape that started from three connected dots. All these secret clues together prove the answer is the four suits of a card deck.