Hint: Transition elements either have partially filled d-orbital or f-orbital. The d-block elements are present starting from the group three to twelve in the periodic table. The position of this block lies between the s block and p block in the periodic table.
Complete answer: Let us first know about transition elements. Transition elements are the elements which have partially filled d-subshells and f-subshells. According to IUPAC, a transition metal is an element which has at least one partially filled d-orbital atom or otherwise can produce cations which have partially filled d-subshells. Primarily, transition elements refer to the d-block elements and the elements of f-block are called inner transition elements. These elements belong to the middle part of the periodic table and are present between the s and p-block elements in the periodic table. The d-block elements are called transition elements because they exhibit transitional behaviour between s-block and p-block elements. Their properties are transitional between highly reactive metallic elements of s-block which are ionic in nature and the elements of p-block which are covalent in nature. Likewise, we can say that the d-block elements construct a bridge between the s and p-block elements.
Note: It is important to note that; zinc, cadmium and mercury are not considered as the transition metals as they consist of fully filled electrons in its d-orbital. Transition metals are small in size and have positive charge density ions.