Which of the following symbols results in a true number sentence when placed in the blank?

The term is used as means of asking students to write down equations using simple mathematical symbols (numerals, the four basic mathematical operators, equality symbol)[5]. Sometimes boxes or shapes are used to indicate unknown values. As such number sentences are used to introduce students to notions of structure and algebra prior to a more formal treatment of these concepts.

A number sentence without unknowns is equivalent to a logical proposition expressed using the notation of arithmetic.

[edit] Examples
A valid number sentence that is true: 3 + 7 = 10.

A valid number sentence that is false: 7 + 9 = 17.

A valid number sentence using a 'less than' symbol: 3 + 6 < 10.

An example from a lesson plan:

Some students will use a direct computational approach. They will carry out the addition 26 + 39 = 65, put 65 = 23 + □, and then find that □ = 42.[6] (wikipedia)

I hope this is helpful!