Is it required to use "that" here?

Solution 1:

A relative pronoun standing at the front of a simple relative clause may be omitted unless it acts as the subject of the relative clause.

^okI contribute to projects that I love.
^okI contribute to projects that support children.

^okI contribute to projects I love. but not, in formal use,
^∗ I contribute to projects support children.

• NOTE: In conversational use you will occasionally hear a subject relative omitted:

  There are some projects support children I will not contribute to.
  I'll ask the man fixes my car about that.

  But this is not acceptable in formal use, and it is often not acceptable in conversation, either. Recognize it when it you hear it; but I advise you not to emulate it.

When there is other matter before the relative pronoun, it may not be omitted:

This is the bag in which I keep my drumsticks. not ^∗the bag in I keep my drumsticks
The exam, passing which is mandatory, is offered in the spring. not ^∗The exam, passing is mandatory ...

And the relative term may not be omitted when it acts as a determiner rather than a pronoun:

The author whose essay you are reading was my teacher. not ^∗The author essay you are reading ...
He advised caution, which advice was unexpected. not ^∗...caution, advice was unexpected.