[caleesirose]

Using the valley of stability model, I get an answer of electron capture or positron emission. However, the answer is beta-decay. *delete me*

[caleesirose]

I calculated n/p= 1.4. The number needs to increase to 1.5, therefore I concluded that the number of neutrons need to increase. Based on this, I concluded that the mode of decay was electron capture or positron emission.

[Enthalpy]

Welcome, caleesirose!

Stable isotopes of indium are 115 and 113. n/p=1.5 is clearly wrong. I don't know if you made some mistake or if said valley stability model is off.
https://www.webelements.com/indium/isotopes.html
¹²⁰In having clearly too many electrons, and being too light for alpha emission, SF and so on, you can suppose that β^- is its main decay mode.

Nuclides tables confirm it:
https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
click on a nuclide for more information.

By the way, β⁺ is also a beta decay, and when a nuclide lacks neutrons, electron capture tends to be rarer than positron emission.

And: if the neutron count were close to the optimum, deductions would be difficult. Think of ⁴⁰K, which decays by β^-, β⁺ and EC.

More generally, models for the stability of nuclides fail. Shell, drop... they all make grossly wrong predictions.