Assuming the tall/short gene in a pea plant follows classic patterns of Mendelian inheritance, the answers would be Tt or TT.
For a pea plant to be short, it needs to inherit two recessive alleles (or alternate forms of a gene). Recessive simply means it won't be expressed if paired with a dominant allele, expressed by a capital letter.
This means, for a pea plant to be tall, it can have inherited two dominant alleles, because tall (T) is dominant over short (t). Because it is also dominant, it can be paired with a recessive allele and will still be expressed. (Tt).
Hope this helps!
-Chrys