Tea (1)/which I am drinking (2)/is hot (3). /No error (4)

Answer

1

Explanation

The will be used before Tea because - Here, we are talking about a particular tea, followed by a Relative Pronoun (which). The sentence can also be written as - The tea I am drinking is hot (tea-definite)
Look at the examples given below:
The flowers in her garden are beautiful. (Definite - the ones in her garden)
The flowers `underset("Rel.pro.")underset(rarr)("(that)")` are in her
`darr`
Rel.. pro.
garden are beautiful.
The dog in the backyard is very cute. (Definite-the one in the backyard)
The dog `underset("Rel.pro.")underset(darr)("(which)")` is in the backyard is very cute. `darr` Rel.pro.
Hence, the tea is the right usage.